diff --git a/interpreter.ts b/interpreter.ts
index a5417cd7..59b0eb09 100644
--- a/interpreter.ts
+++ b/interpreter.ts
@@ -82,17 +82,14 @@ module J2ME {
    * Debugging helper to make sure native methods were implemented correctly.
    */
   function checkReturnValue(methodInfo: MethodInfo, returnValue: any) {
-    if (returnValue instanceof Promise) {
-      console.error("You forgot to call asyncImpl():", methodInfo.implKey);
-    } else if (methodInfo.getReturnKind() === Kind.Void && returnValue) {
-      console.error("You returned something in a void method:", methodInfo.implKey);
-    } else if (methodInfo.getReturnKind() !== Kind.Void && (returnValue === undefined) &&
-      !U) {
-      console.error("You returned undefined in a non-void method:", methodInfo.implKey);
-    } else if (typeof returnValue === "string") {
-      console.error("You returned a non-wrapped string:", methodInfo.implKey);
-    } else if (returnValue === true || returnValue === false) {
-      console.error("You returned a JS boolean:", methodInfo.implKey);
+    if (U) {
+      if (typeof returnValue !== "undefined") {
+        assert(false, "Expected undefined return value during unwind, got " + returnValue);
+      }
+      return;
+    }
+    if (!(getKindCheck(methodInfo.getReturnKind())(returnValue))) {
+      assert(false, "Expected " + Kind[methodInfo.getReturnKind()] + " return value, got " + returnValue);
     }
   }
 
diff --git a/libs/long.js b/libs/long.js
index c019ea2e..f3a10438 100644
--- a/libs/long.js
+++ b/libs/long.js
@@ -659,4 +659,5 @@
     exports.fromNumber = fromNumber;
     exports.fromBits = fromBits;
     exports.fromString = fromString;
+    exports.constructor = Long;
 })(typeof exports === "undefined" ? this.Long = {} : exports);
diff --git a/types.ts b/types.ts
index a7ac8a53..f0712a0c 100644
--- a/types.ts
+++ b/types.ts
@@ -102,11 +102,11 @@ module J2ME {
       case Kind.Int:
         return (x) => (x | 0) === x;
       case Kind.Float:
-        return (x) => Math.fround(x) === x;
+        return (x) => isNaN(x) || Math.fround(x) === x;
       case Kind.Long:
-        return (x) => x instanceof Long;
+        return (x) => x instanceof Long.constructor;
       case Kind.Double:
-        return (x) => (+x) === x;
+        return (x) => isNaN(x) || (+x) === x;
       case Kind.Reference:
         return (x) => x === null || x instanceof Object;
       case Kind.Void: