diff --git a/src/ProtocolBuffers.Test/TestWriterFormatJson.cs b/src/ProtocolBuffers.Test/TestWriterFormatJson.cs
index b658086cd9..6b163824cb 100644
--- a/src/ProtocolBuffers.Test/TestWriterFormatJson.cs
+++ b/src/ProtocolBuffers.Test/TestWriterFormatJson.cs
@@ -492,7 +492,7 @@ namespace Google.ProtocolBuffers
                .SetPackage("package")
                .BuildPartial();
 
-            Assert.NotNull(opts.ToJson());
+            Assert.IsNotNull(opts.ToJson());
         }
     }
 }