diff --git a/src/qml/compiler/qqmlirbuilder.cpp b/src/qml/compiler/qqmlirbuilder.cpp
index 4a1b27d7aa..4e87c96b68 100644
--- a/src/qml/compiler/qqmlirbuilder.cpp
+++ b/src/qml/compiler/qqmlirbuilder.cpp
@@ -1824,6 +1824,9 @@ QVector<int> JSCodeGen::generateJSCodeForFunctionsAndBindings(const QList<Compil
     }
     scan.leaveEnvironment();
 
+    if (hasError)
+        return QVector<int>();
+
     _context = nullptr;
 
     for (int i = 0; i < functions.count(); ++i) {
diff --git a/src/qml/compiler/qv4codegen.cpp b/src/qml/compiler/qv4codegen.cpp
index 52dd32dcc5..b891b5bbad 100644
--- a/src/qml/compiler/qv4codegen.cpp
+++ b/src/qml/compiler/qv4codegen.cpp
@@ -120,6 +120,9 @@ void Codegen::generateFromProgram(const QString &fileName,
     ScanFunctions scan(this, sourceCode, mode);
     scan(node);
 
+    if (hasError)
+        return;
+
     defineFunction(QStringLiteral("%entry"), node, nullptr, node->elements);
 }
 
@@ -2210,16 +2213,6 @@ static bool endsWithReturn(Node *node)
     return false;
 }
 
-static bool isSimpleParameterList(AST::FormalParameterList *formals)
-{
-    while (formals) {
-        if (formals->isRest || formals->defaultExpression)
-            return false;
-        formals = formals->next;
-    }
-    return true;
-}
-
 int Codegen::defineFunction(const QString &name, AST::Node *ast,
                             AST::FormalParameterList *formals,
                             AST::SourceElements *body)
@@ -2324,7 +2317,7 @@ int Codegen::defineFunction(const QString &name, AST::Node *ast,
         }
     }
     if (_context->usesArgumentsObject == Context::ArgumentsObjectUsed) {
-        if (_context->isStrict || !isSimpleParameterList(formals)) {
+        if (_context->isStrict || !formals->isSimpleParameterList()) {
             Instruction::CreateUnmappedArgumentsObject setup;
             bytecodeGenerator->addInstruction(setup);
         } else {
diff --git a/src/qml/compiler/qv4compilerscanfunctions.cpp b/src/qml/compiler/qv4compilerscanfunctions.cpp
index f985c74f9d..8824a47def 100644
--- a/src/qml/compiler/qv4compilerscanfunctions.cpp
+++ b/src/qml/compiler/qv4compilerscanfunctions.cpp
@@ -431,12 +431,14 @@ void ScanFunctions::enterFunction(Node *ast, const QString &name, FormalParamete
     if (body && !_context->isStrict)
         checkDirectivePrologue(body->elements);
 
+    bool isSimpleParameterList = formals->isSimpleParameterList();
+
     for (FormalParameterList *it = formals; it; it = it->next) {
         QString arg = it->name.toString();
         int duplicateIndex = _context->arguments.indexOf(arg);
         if (duplicateIndex != -1) {
-            if (_context->isStrict) {
-                _cg->throwSyntaxError(it->identifierToken, QStringLiteral("Duplicate parameter name '%1' is not allowed in strict mode").arg(arg));
+            if (_context->isStrict || !isSimpleParameterList) {
+                _cg->throwSyntaxError(it->identifierToken, QStringLiteral("Duplicate parameter name '%1' is not allowed.").arg(arg));
                 return;
             } else {
                 // change the name of the earlier argument to enforce the specified lookup semantics
diff --git a/src/qml/jsruntime/qv4functionobject.cpp b/src/qml/jsruntime/qv4functionobject.cpp
index 73a8eb1193..390e0d5161 100644
--- a/src/qml/jsruntime/qv4functionobject.cpp
+++ b/src/qml/jsruntime/qv4functionobject.cpp
@@ -240,6 +240,9 @@ ReturnedValue FunctionCtor::callAsConstructor(const FunctionObject *f, const Val
     RuntimeCodegen cg(scope.engine, &jsGenerator, false);
     cg.generateFromFunctionExpression(QString(), function, fe, &module);
 
+    if (scope.hasException())
+        return Encode::undefined();
+
     QQmlRefPointer<CompiledData::CompilationUnit> compilationUnit = cg.generateCompilationUnit();
     Function *vmf = compilationUnit->linkToEngine(scope.engine);
 
diff --git a/src/qml/jsruntime/qv4runtimecodegen.cpp b/src/qml/jsruntime/qv4runtimecodegen.cpp
index fe18ddf9ed..8080ef7344 100644
--- a/src/qml/jsruntime/qv4runtimecodegen.cpp
+++ b/src/qml/jsruntime/qv4runtimecodegen.cpp
@@ -58,6 +58,9 @@ void RuntimeCodegen::generateFromFunctionExpression(const QString &fileName,
     scan(ast);
     scan.leaveEnvironment();
 
+    if (hasError)
+        return;
+
     int index = defineFunction(ast->name.toString(), ast, ast->formals, ast->body ? ast->body->elements : nullptr);
     _module->rootContext = _module->functions.at(index);
 }
diff --git a/src/qml/parser/qqmljsast_p.h b/src/qml/parser/qqmljsast_p.h
index 61ee89ad2f..51f3d78588 100644
--- a/src/qml/parser/qqmljsast_p.h
+++ b/src/qml/parser/qqmljsast_p.h
@@ -2099,6 +2099,18 @@ public:
         return n;
     }
 
+    bool isSimpleParameterList()
+    {
+        AST::FormalParameterList *formals = this;
+        while (formals) {
+            if (formals->isRest || formals->defaultExpression)
+                return false;
+            formals = formals->next;
+        }
+        return true;
+    }
+
+
     void accept0(Visitor *visitor) override;
 
     SourceLocation firstSourceLocation() const override