Throwing a helpful error in cases where the user forgot to use the TypeToken idiom.

gson/src/main/java/com/google/gson/TypeInfoFactory.java

@@ -80,6 +80,7 @@ final class TypeInfoFactory {
         }
       }
     } else if (typeToEvaluate instanceof TypeVariable) {
+      if (parentType instanceof ParameterizedType) {
         // The class definition has the actual types used for the type variables.
         // Find the matching actual type for the Type Variable used for the field.
         // For example, class Foo<A> { A a; }
@@ -92,6 +93,11 @@ final class TypeInfoFactory {
         int indexOfActualTypeArgument = getIndex(classTypeVariables, fieldTypeVariable);
         Type[] actualTypeArguments = objParameterizedType.getActualTypeArguments();
         return actualTypeArguments[indexOfActualTypeArgument];
+      } else {
+        throw new UnsupportedOperationException("Expecting parameterized type, got " + parentType
+            + ".\n Are you missing the use of TypeToken idiom?\n See " 
+            + "http://sites.google.com/site/gson/gson-user-guide#TOC-Serializing-and-Deserializing-Gener");
+      }
     } else if (typeToEvaluate instanceof WildcardType) {
       WildcardType castedType = (WildcardType) typeToEvaluate;
       return getActualType(castedType.getUpperBounds()[0], parentType, rawParentClass);