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 0fe193a..bb23493 100644
--- a/user/super/com/google/gwt/emul/java/lang/String.java
+++ b/user/super/com/google/gwt/emul/java/lang/String.java
@@ -27,12 +27,15 @@
 import java.util.Locale;
 import java.util.StringJoiner;
 
+import javax.annotation.Nonnull;
+
 import javaemul.internal.ArrayHelper;
 import javaemul.internal.EmulatedCharset;
 import javaemul.internal.HashCodes;
 import javaemul.internal.JsUtils;
 import javaemul.internal.NativeRegExp;
 import javaemul.internal.annotations.DoNotInline;
+
 import jsinterop.annotations.JsMethod;
 import jsinterop.annotations.JsPackage;
 import jsinterop.annotations.JsProperty;
@@ -186,6 +189,7 @@
 
   // valueOf needs to be treated special:
   // J2cl uses it for String concat and thus it can not use string concatenation itself.
+  @Nonnull
   public static String valueOf(Object x) {
     return x == null ? "null" : x.toString();
   }