diff --git a/src/libraries/Common/tests/TestUtilities/System/PlatformDetection.cs b/src/libraries/Common/tests/TestUtilities/System/PlatformDetection.cs
index 510b4b8e32180..210e8b6fc99f7 100644
--- a/src/libraries/Common/tests/TestUtilities/System/PlatformDetection.cs
+++ b/src/libraries/Common/tests/TestUtilities/System/PlatformDetection.cs
@@ -136,6 +136,7 @@ public static int SlowRuntimeTimeoutModifier
         public static bool IsWasmThreadingSupported => IsBrowser && IsEnvironmentVariableTrue("IsBrowserThreadingSupported");
         public static bool IsNotWasmThreadingSupported => !IsWasmThreadingSupported;
         public static bool IsWasmBackgroundExec => IsBrowser && IsEnvironmentVariableTrue("IsWasmBackgroundExec");
+        public static bool IsThreadingSupportedNotBrowserBackgroundExec => IsWasmThreadingSupported && !IsWasmBackgroundExec;
         public static bool IsWasmBackgroundExecOrSingleThread => IsWasmBackgroundExec || IsNotWasmThreadingSupported;
         public static bool IsThreadingSupportedOrBrowserBackgroundExec => IsWasmBackgroundExec || !IsBrowser;
         public static bool IsBinaryFormatterSupported => IsNotMobile && !IsNativeAot;
diff --git a/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSFunctionBinding.cs b/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSFunctionBinding.cs
index 618c5e02e0d05..550956cce3309 100644
--- a/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSFunctionBinding.cs
+++ b/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSFunctionBinding.cs
@@ -207,7 +207,7 @@ internal static unsafe void InvokeJSFunction(JSObject jsFunction, Span<JSMarshal
 
 #if FEATURE_WASM_MANAGED_THREADS
             ref JSMarshalerArgument exc = ref arguments[0];
-            exc.slot.CallerNativeTID = jsFunction.ProxyContext.NativeTID;
+            exc.slot.CallerNativeTID = JSProxyContext.GetNativeThreadId();
 
             // if we are on correct thread already, just call it
             if (jsFunction.ProxyContext.IsCurrentThread())
@@ -297,7 +297,7 @@ internal static unsafe void InvokeJSImportImpl(JSFunctionBinding signature, Span
             ref JSMarshalerArgument res = ref arguments[1];
 #if FEATURE_WASM_MANAGED_THREADS
             var targetContext = JSProxyContext.SealJSImportCapturing();
-            exc.slot.CallerNativeTID = targetContext.NativeTID;
+            exc.slot.CallerNativeTID = JSProxyContext.GetNativeThreadId();
             exc.slot.ContextHandle = targetContext.ContextHandle;
             res.slot.ContextHandle = targetContext.ContextHandle;
 #else
@@ -467,7 +467,7 @@ internal static unsafe void ResolveOrRejectPromise(JSProxyContext targetContext,
         {
             ref JSMarshalerArgument exc = ref arguments[0];
 #if FEATURE_WASM_MANAGED_THREADS
-            exc.slot.CallerNativeTID = targetContext.NativeTID;
+            exc.slot.CallerNativeTID = JSProxyContext.GetNativeThreadId();
 
             if (targetContext.IsCurrentThread())
 #endif
diff --git a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSExportTest.cs b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSExportTest.cs
index 8fe5e467eca06..2d7b7cbf6429c 100644
--- a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSExportTest.cs
+++ b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSExportTest.cs
@@ -14,6 +14,13 @@ namespace System.Runtime.InteropServices.JavaScript.Tests
 {
     public class JSExportAsyncTest : JSInteropTestBase, IAsyncLifetime
     {
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsThreadingSupportedNotBrowserBackgroundExec))]
+        public void SyncJsImportJsExportThrows()
+        {
+            var ex = Assert.Throws<JSException>(()=>JavaScriptTestHelper.invoke1_Boolean(true, nameof(JavaScriptTestHelper.EchoBoolean)));
+            Assert.Contains("Cannot call synchronous C# method", ex.Message);
+        }
+
         [Theory]
         [MemberData(nameof(MarshalBooleanCases))]
         public async Task JsExportBooleanAsync(bool value)
@@ -150,6 +157,7 @@ public void JsExportIntPtr(IntPtr value)
         [MemberData(nameof(MarshalIntPtrCases))]
         public unsafe void JsExportVoidPtr(IntPtr xvalue)
         {
+            JavaScriptTestHelper.AssertWasmBackgroundExec();
             void* value = (void*)xvalue;
             void* res = JavaScriptTestHelper.invoke1_VoidPtr(value, nameof(JavaScriptTestHelper.EchoVoidPtr));
             Assert.True(value == res);
@@ -180,6 +188,7 @@ public void JsExportDateTimeOffset(DateTimeOffset value)
         [MemberData(nameof(MarshalNullableBooleanCases))]
         public void JsExportNullableBoolean(bool? value)
         {
+            JavaScriptTestHelper.AssertWasmBackgroundExec();
             JsExportTest(value,
                 JavaScriptTestHelper.invoke1_NullableBoolean,
                 nameof(JavaScriptTestHelper.EchoNullableBoolean),
diff --git a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSImportTest.cs b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSImportTest.cs
index 89ec5cbb08978..f3ce777e82456 100644
--- a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSImportTest.cs
+++ b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSImportTest.cs
@@ -116,6 +116,7 @@ public unsafe void OutOfRange()
         [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsWasmBackgroundExecOrSingleThread))]
         public unsafe void OptimizedPaths()
         {
+            JavaScriptTestHelper.AssertWasmBackgroundExec();
             JavaScriptTestHelper.optimizedReached = 0;
             JavaScriptTestHelper.invoke0V();
             Assert.Equal(1, JavaScriptTestHelper.optimizedReached);
diff --git a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JavaScriptTestHelper.cs b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JavaScriptTestHelper.cs
index e5ea3a887e24b..232397defef1d 100644
--- a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JavaScriptTestHelper.cs
+++ b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JavaScriptTestHelper.cs
@@ -1020,14 +1020,24 @@ public static JSObject EchoIJSObject([JSMarshalAs<JSType.Object>] JSObject arg1)
         [JSImport("INTERNAL.forceDisposeProxies")]
         internal static partial void ForceDisposeProxies(bool disposeMethods, bool verbose);
 
+        public static void AssertWasmBackgroundExec()
+        {
+            if (PlatformDetection.IsWasmBackgroundExec && Environment.CurrentManagedThreadId == 1)
+            {
+                throw new Exception("With WasmBackgroundExec we are expecting to run tests on the thread pool");
+            }
+        }
+
         static JSObject _module;
         public static async Task InitializeAsync()
         {
+            AssertWasmBackgroundExec();
             if (_module == null)
             {
                 _module = await JSHost.ImportAsync("JavaScriptTestHelper", "../JavaScriptTestHelper.mjs"); ;
                 await Setup();
             }
+            AssertWasmBackgroundExec();
 
 #if FEATURE_WASM_MANAGED_THREADS
             // are we in the UI thread ?
@@ -1037,6 +1047,7 @@ public static async Task InitializeAsync()
                 // this gives browser chance to serve UI thread event loop before every test
                 await Task.Yield();
             }
+            AssertWasmBackgroundExec();
         }
 
         public static Task DisposeAsync()
diff --git a/src/mono/browser/runtime/invoke-js.ts b/src/mono/browser/runtime/invoke-js.ts
index b2a352cf5e94e..c13250fbe0ba4 100644
--- a/src/mono/browser/runtime/invoke-js.ts
+++ b/src/mono/browser/runtime/invoke-js.ts
@@ -142,7 +142,7 @@ function bind_js_import(signature: JSFunctionSignature): Function {
         try {
             forceThreadMemoryViewRefresh();
             const caller_tid = get_caller_native_tid(args);
-            runtimeHelpers.isPendingSynchronousCall = runtimeHelpers.currentThreadTID === caller_tid;
+            runtimeHelpers.isPendingSynchronousCall = runtimeHelpers.managedThreadTID === caller_tid;
             bound_fn(args);
         }
         finally {