diff --git a/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TObject.java b/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TObject.java
index ebad4005b..cf2cd483e 100644
--- a/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TObject.java
+++ b/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TObject.java
@@ -216,6 +216,9 @@ public class TObject {
 
     @Rename("wait")
     public final void wait0(long timeout, int nanos, final AsyncCallback<Void> callback) {
+        if (!holdsLock(this)) {
+            throw new TIllegalMonitorStateException();
+        }
         final NotifyListenerImpl listener = new NotifyListenerImpl(this, callback, monitor.count);
         monitor.notifyListeners.add(listener);
         if (timeout > 0 || nanos > 0) {
diff --git a/teavm-core/src/main/java/org/teavm/javascript/Renderer.java b/teavm-core/src/main/java/org/teavm/javascript/Renderer.java
index b28568e48..195cf3041 100644
--- a/teavm-core/src/main/java/org/teavm/javascript/Renderer.java
+++ b/teavm-core/src/main/java/org/teavm/javascript/Renderer.java
@@ -76,7 +76,7 @@ public class Renderer implements ExprVisitor, StatementVisitor, RenderingContext
         try {
             MethodReference monitorEnterRef = new MethodReference(
                     Object.class, "monitorEnter", Object.class, void.class);
-            writer.appendMethodBody(monitorEnterRef).append("(");
+            writer.append("return ").appendMethodBody(monitorEnterRef).append("(");
             statement.getObjectRef().acceptVisitor(this);
             writer.append(",").ws();
             writer.append("$rt_continue($part_").append(statement.getAsyncTarget()).append(')');
diff --git a/teavm-platform/src/main/java/org/teavm/platform/plugin/AsyncMethodGenerator.java b/teavm-platform/src/main/java/org/teavm/platform/plugin/AsyncMethodGenerator.java
index 68ae8a10a..d81151af1 100644
--- a/teavm-platform/src/main/java/org/teavm/platform/plugin/AsyncMethodGenerator.java
+++ b/teavm-platform/src/main/java/org/teavm/platform/plugin/AsyncMethodGenerator.java
@@ -39,12 +39,12 @@ public class AsyncMethodGenerator implements Generator, DependencyPlugin {
     public void generate(GeneratorContext context, SourceWriter writer, MethodReference methodRef) throws IOException {
         MethodReference asyncRef = getAsyncReference(methodRef);
         writer.append("var callback").ws().append("=").ws().append("function()").ws().append("{};").softNewLine();
-        writer.append("callback.").appendMethod(completeMethod).ws().append("=").ws().append("function($this,").ws()
-                .append("val)").ws().append("{").indent().softNewLine();
+        writer.append("callback.").appendMethod(completeMethod).ws().append("=").ws().append("function(val)").ws()
+                .append("{").indent().softNewLine();
         writer.append("return $return($rt_asyncResult(val));").softNewLine();
         writer.outdent().append("};").softNewLine();
-        writer.append("callback.").appendMethod(errorMethod).ws().append("=").ws().append("function($this,").ws()
-                .append("e)").ws().append("{").indent().softNewLine();
+        writer.append("callback.").appendMethod(errorMethod).ws().append("=").ws().append("function(e)").ws()
+                .append("{").indent().softNewLine();
         writer.append("return $return($rt_asyncError(e));").softNewLine();
         writer.outdent().append("};").softNewLine();
         writer.append("try").ws().append("{").indent().softNewLine();