diff --git a/user/super/com/google/gwt/emul/java/lang/String.java b/user/super/com/google/gwt/emul/java/lang/String.java
index cfe2586..6421e68 100644
--- a/user/super/com/google/gwt/emul/java/lang/String.java
+++ b/user/super/com/google/gwt/emul/java/lang/String.java
@@ -17,6 +17,7 @@
 package java.lang;
 
 import static javaemul.internal.InternalPreconditions.checkCriticalStringBounds;
+import static javaemul.internal.InternalPreconditions.checkNotNull;
 
 import java.io.Serializable;
 import java.io.UnsupportedEncodingException;
@@ -750,7 +751,7 @@
      * Magic: this method is only used during compiler optimizations; the generated JS will instead alias
      * this method to the native String.prototype.toString() function.
      */
-    return this;
+    return checkNotNull(this);
   }
 
   public String trim() {
diff --git a/user/super/com/google/gwt/emul/java/util/StringJoiner.java b/user/super/com/google/gwt/emul/java/util/StringJoiner.java
index 6071c03..c93b3b4 100644
--- a/user/super/com/google/gwt/emul/java/util/StringJoiner.java
+++ b/user/super/com/google/gwt/emul/java/util/StringJoiner.java
@@ -35,11 +35,6 @@
   }
 
   public StringJoiner(CharSequence delimiter, CharSequence prefix, CharSequence suffix) {
-    // TODO: null.toString() does not throw exception
-    checkCriticalNotNull(delimiter, "delimiter");
-    checkCriticalNotNull(prefix, "prefix");
-    checkCriticalNotNull(suffix, "suffix");
-
     this.delimiter = delimiter.toString();
     this.prefix = prefix.toString();
     this.suffix = suffix.toString();
@@ -96,4 +91,4 @@
       builder.append(delimiter);
     }
   }
-}
\ No newline at end of file
+}