diff --git a/user/src/com/google/gwt/resources/css/SubstitutionReplacer.java b/user/src/com/google/gwt/resources/css/SubstitutionReplacer.java
index 0fe893f..fdbd901 100644
--- a/user/src/com/google/gwt/resources/css/SubstitutionReplacer.java
+++ b/user/src/com/google/gwt/resources/css/SubstitutionReplacer.java
@@ -109,25 +109,21 @@
           throw new CssCompilerException("Cannot find data method");
         }
 
-        if (!methodType.equals(dataResourceType) &&
-            !methodType.equals(imageResourceType)) {
+        if (!(methodType instanceof JClassType) ||
+            (!dataResourceType.isAssignableFrom((JClassType) methodType) &&
+            !imageResourceType.isAssignableFrom((JClassType) methodType))) {
           String message = "Invalid method type for url substitution: " + methodType + ". " +
               "Only DataResource and ImageResource are supported.";
           logger.log(TreeLogger.ERROR, message);
           throw new CssCompilerException(message);
         }
 
-        String instance = "((" + methodType.getQualifiedSourceName() + ")("
-            + context.getImplementationSimpleSourceName() + ".this."
-            + functionName.getExpression() + "))";
-
         StringBuilder expression = new StringBuilder();
         expression.append("\"url('\" + ");
-        if (methodType.equals(dataResourceType)) {
-          expression.append(instance).append(".getUrl()");
-        } else if (methodType.equals(imageResourceType)) {
-          expression.append(instance).append(".getURL()");
-        }
+        expression.append(context.getImplementationSimpleSourceName());
+        expression.append(".this.");
+        expression.append(functionName.getExpression());
+        expression.append(".getSafeUri().asString()");
         expression.append(" + \"')\"");
         result.add(new ExpressionValue(expression.toString()));
       } else {
diff --git a/user/test/com/google/gwt/resources/client/CSSResourceTest.java b/user/test/com/google/gwt/resources/client/CSSResourceTest.java
index be907ae..d4daa15 100644
--- a/user/test/com/google/gwt/resources/client/CSSResourceTest.java
+++ b/user/test/com/google/gwt/resources/client/CSSResourceTest.java
@@ -21,6 +21,7 @@
 import com.google.gwt.resources.client.CssResource.ImportedWithPrefix;
 import com.google.gwt.resources.client.CssResource.Shared;
 
+
 /**
  * Contains various full-stack tests of the CssResource system.
  */
@@ -186,6 +187,12 @@
     FullTestCss css();
 
     @Source("32x32.png")
+    CustomDataResource customDataMethod();
+
+    @Source("16x16.png")
+    CustomImageResource customSpriteMethod();
+
+    @Source("32x32.png")
     DataResource dataMethod();
 
     // Test default extensions
@@ -310,13 +317,17 @@
 
     // Check data URL expansion
     assertTrue(text.contains("backgroundTopLevel:url('"
-        + Resources.INSTANCE.dataMethod().getUrl() + "')"));
+        + Resources.INSTANCE.dataMethod().getSafeUri().asString() + "')"));
     assertTrue(text.contains("backgroundNested:url('"
-        + Resources.INSTANCE.nested().dataMethod().getUrl() + "')"));
+        + Resources.INSTANCE.nested().dataMethod().getSafeUri().asString() + "')"));
+    assertTrue(text.contains("backgroundCustom:url('"
+        + Resources.INSTANCE.customDataMethod().getSafeUri().asString() + "')"));
     assertTrue(text.contains("backgroundImage:url('"
-        + Resources.INSTANCE.spriteMethod().getURL() + "')"));
+        + Resources.INSTANCE.spriteMethod().getSafeUri().asString() + "')"));
     assertTrue(text.contains("backgroundImageNested:url('"
-        + Resources.INSTANCE.nested().spriteMethod().getURL() + "')"));
+        + Resources.INSTANCE.nested().spriteMethod().getSafeUri().asString() + "')"));
+    assertTrue(text.contains("backgroundImageCustom:url('"
+        + Resources.INSTANCE.customSpriteMethod().getSafeUri().asString() + "')"));
 
     // Check @eval expansion
     assertTrue(text.contains(red() + ";"));
diff --git a/user/test/com/google/gwt/resources/client/CustomDataResource.java b/user/test/com/google/gwt/resources/client/CustomDataResource.java
new file mode 100644
index 0000000..6658a61
--- /dev/null
+++ b/user/test/com/google/gwt/resources/client/CustomDataResource.java
@@ -0,0 +1,26 @@
+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package com.google.gwt.resources.client;
+
+import com.google.gwt.resources.ext.ResourceGeneratorType;
+import com.google.gwt.resources.rg.CustomDataResourceGenerator;
+
+/**
+ * Custom subtype of {@link DataResource}, to test {@code @url} in CssResource.
+ */
+@ResourceGeneratorType(CustomDataResourceGenerator.class)
+public interface CustomDataResource extends DataResource {
+}
diff --git a/user/test/com/google/gwt/resources/client/CustomImageResource.java b/user/test/com/google/gwt/resources/client/CustomImageResource.java
new file mode 100644
index 0000000..84e067a
--- /dev/null
+++ b/user/test/com/google/gwt/resources/client/CustomImageResource.java
@@ -0,0 +1,26 @@
+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package com.google.gwt.resources.client;
+
+import com.google.gwt.resources.ext.ResourceGeneratorType;
+import com.google.gwt.resources.rg.CustomImageResourceGenerator;
+
+/**
+ * Custom subtype of {@link ImageResource}, to test {@code @url} in CssResource.
+ */
+@ResourceGeneratorType(CustomImageResourceGenerator.class)
+public interface CustomImageResource extends ImageResource {
+}
diff --git a/user/test/com/google/gwt/resources/client/impl/CustomDataResourcePrototype.java b/user/test/com/google/gwt/resources/client/impl/CustomDataResourcePrototype.java
new file mode 100644
index 0000000..f8c65fd
--- /dev/null
+++ b/user/test/com/google/gwt/resources/client/impl/CustomDataResourcePrototype.java
@@ -0,0 +1,28 @@
+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package com.google.gwt.resources.client.impl;
+
+import com.google.gwt.resources.client.CustomDataResource;
+import com.google.gwt.safehtml.shared.SafeUri;
+
+/**
+ * Default implementation of {@link CustomDataResource}.
+ */
+public class CustomDataResourcePrototype extends DataResourcePrototype implements CustomDataResource {
+  public CustomDataResourcePrototype(String name, SafeUri uri) {
+    super(name, uri);
+  }
+}
diff --git a/user/test/com/google/gwt/resources/client/impl/CustomImageResourcePrototype.java b/user/test/com/google/gwt/resources/client/impl/CustomImageResourcePrototype.java
new file mode 100644
index 0000000..9bcbc8e
--- /dev/null
+++ b/user/test/com/google/gwt/resources/client/impl/CustomImageResourcePrototype.java
@@ -0,0 +1,30 @@
+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package com.google.gwt.resources.client.impl;
+
+import com.google.gwt.resources.client.CustomImageResource;
+import com.google.gwt.safehtml.shared.SafeUri;
+
+/**
+ * Default implementation of {@link CustomImageResource}.
+ */
+public class CustomImageResourcePrototype extends ImageResourcePrototype implements
+    CustomImageResource {
+
+  public CustomImageResourcePrototype(String name, SafeUri url) {
+    super(name, url, 0, 0, 0, 0, false, false);
+  }
+}
diff --git a/user/test/com/google/gwt/resources/client/test.css b/user/test/com/google/gwt/resources/client/test.css
index f93d24c..f5e6c4a 100644
--- a/user/test/com/google/gwt/resources/client/test.css
+++ b/user/test/com/google/gwt/resources/client/test.css
@@ -18,8 +18,10 @@
 @eval RED com.google.gwt.resources.client.CSSResourceTest.red();
 @url BACKGROUND dataMethod;
 @url NESTEDBACKGROUND nested.dataMethod;
+@url CUSTOMBACKGROUND customDataMethod;
 @url BACKGROUNDIMAGE spriteMethod;
 @url NESTEDBACKGROUNDIMAGE nested.spriteMethod;
+@url CUSTOMBACKGROUNDIMAGE customSpriteMethod;
 
 /* Check @def expansion */
 @def SPRITEWIDTH value("spriteMethod.getWidth", "px");
@@ -59,8 +61,10 @@
     direction: DIRECTION;
     backgroundTopLevel: BACKGROUND;
     backgroundNested: NESTEDBACKGROUND;
+    backgroundCustom: CUSTOMBACKGROUND;
     backgroundImage: BACKGROUNDIMAGE;
     backgroundImageNested: NESTEDBACKGROUNDIMAGE;
+    backgroundImageCustom: CUSTOMBACKGROUNDIMAGE;
 }
 
 div[foo="bar"] {
diff --git a/user/test/com/google/gwt/resources/rg/CustomDataResourceGenerator.java b/user/test/com/google/gwt/resources/rg/CustomDataResourceGenerator.java
new file mode 100644
index 0000000..6c33af4
--- /dev/null
+++ b/user/test/com/google/gwt/resources/rg/CustomDataResourceGenerator.java
@@ -0,0 +1,58 @@
+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package com.google.gwt.resources.rg;
+
+import com.google.gwt.core.ext.TreeLogger;
+import com.google.gwt.core.ext.UnableToCompleteException;
+import com.google.gwt.core.ext.typeinfo.JMethod;
+import com.google.gwt.resources.client.impl.CustomDataResourcePrototype;
+import com.google.gwt.resources.ext.AbstractResourceGenerator;
+import com.google.gwt.resources.ext.ResourceContext;
+import com.google.gwt.resources.ext.ResourceGeneratorUtil;
+import com.google.gwt.safehtml.shared.UriUtils;
+import com.google.gwt.user.rebind.SourceWriter;
+import com.google.gwt.user.rebind.StringSourceWriter;
+
+import java.net.URL;
+
+/**
+ * Generator for {@link CustomDataResource}.
+ */
+public class CustomDataResourceGenerator extends AbstractResourceGenerator {
+  @Override
+  public String createAssignment(TreeLogger logger, ResourceContext context, JMethod method)
+      throws UnableToCompleteException {
+    URL[] resources = ResourceGeneratorUtil.findResources(logger, context, method);
+
+    if (resources.length != 1) {
+      logger.log(TreeLogger.ERROR, "Exactly one resource must be specified", null);
+      throw new UnableToCompleteException();
+    }
+
+    URL resource = resources[0];
+
+    SourceWriter sw = new StringSourceWriter();
+    sw.println("new " + CustomDataResourcePrototype.class.getName() + "(");
+    sw.indent();
+    sw.println('"' + method.getName() + "\",");
+    // We don't care about it actually working, so just use the resource URL
+    sw.println(UriUtils.class.getName() + ".fromTrustedString(\"" + resource.toExternalForm() + "\")");
+    sw.outdent();
+    sw.print(")");
+
+    return sw.toString();
+  }
+}
diff --git a/user/test/com/google/gwt/resources/rg/CustomImageResourceGenerator.java b/user/test/com/google/gwt/resources/rg/CustomImageResourceGenerator.java
new file mode 100644
index 0000000..357d652
--- /dev/null
+++ b/user/test/com/google/gwt/resources/rg/CustomImageResourceGenerator.java
@@ -0,0 +1,59 @@
+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package com.google.gwt.resources.rg;
+
+import com.google.gwt.core.ext.TreeLogger;
+import com.google.gwt.core.ext.UnableToCompleteException;
+import com.google.gwt.core.ext.typeinfo.JMethod;
+import com.google.gwt.resources.client.impl.CustomImageResourcePrototype;
+import com.google.gwt.resources.ext.AbstractResourceGenerator;
+import com.google.gwt.resources.ext.ResourceContext;
+import com.google.gwt.resources.ext.ResourceGeneratorUtil;
+import com.google.gwt.safehtml.shared.UriUtils;
+import com.google.gwt.user.rebind.SourceWriter;
+import com.google.gwt.user.rebind.StringSourceWriter;
+
+import java.net.URL;
+
+/**
+ * Generator for {@link CustomImageResource}.
+ */
+public class CustomImageResourceGenerator extends AbstractResourceGenerator {
+
+  @Override
+  public String createAssignment(TreeLogger logger, ResourceContext context, JMethod method)
+      throws UnableToCompleteException {
+    URL[] resources = ResourceGeneratorUtil.findResources(logger, context, method);
+
+    if (resources.length != 1) {
+      logger.log(TreeLogger.ERROR, "Exactly one resource must be specified", null);
+      throw new UnableToCompleteException();
+    }
+
+    URL resource = resources[0];
+
+    SourceWriter sw = new StringSourceWriter();
+    sw.println("new " + CustomImageResourcePrototype.class.getName() + "(");
+    sw.indent();
+    sw.println('"' + method.getName() + "\",");
+    // We don't care about it actually working, so just use the resource URL
+    sw.println(UriUtils.class.getName() + ".fromTrustedString(\"" + resource.toExternalForm() + "\")");
+    sw.outdent();
+    sw.print(")");
+
+    return sw.toString();
+  }
+}