diff --git a/modal/partial_function.py b/modal/partial_function.py
index e49ba8487..13302026f 100644
--- a/modal/partial_function.py
+++ b/modal/partial_function.py
@@ -379,6 +379,11 @@ def wrapper(raw_f: Callable[..., Any]) -> _PartialFunction:
                     f"Modal will drop support for default parameters in a future release.",
                 )
 
+        if inspect.iscoroutinefunction(raw_f):
+            raise InvalidError(
+                f"ASGI app function {raw_f.__name__} is an async function. Only sync Python functions are supported."
+            )
+
         if not wait_for_response:
             deprecation_error(
                 (2024, 5, 13),
@@ -448,6 +453,11 @@ def wrapper(raw_f: Callable[..., Any]) -> _PartialFunction:
                     f"Modal will drop support for default parameters in a future release.",
                 )
 
+        if inspect.iscoroutinefunction(raw_f):
+            raise InvalidError(
+                f"WSGI app function {raw_f.__name__} is an async function. Only sync Python functions are supported."
+            )
+
         if not wait_for_response:
             deprecation_error(
                 (2024, 5, 13),
diff --git a/test/webhook_test.py b/test/webhook_test.py
index 8bb123a56..dd343ec27 100644
--- a/test/webhook_test.py
+++ b/test/webhook_test.py
@@ -134,40 +134,54 @@ async def test_asgi_wsgi(servicer, client):
 
     @app.function(serialized=True)
     @asgi_app()
-    async def my_asgi():
+    def my_asgi():
         pass
 
     @app.function(serialized=True)
     @wsgi_app()
-    async def my_wsgi():
+    def my_wsgi():
         pass
 
     with pytest.raises(InvalidError, match="can't have parameters"):
 
         @app.function(serialized=True)
         @asgi_app()
-        async def my_invalid_asgi(x):
+        def my_invalid_asgi(x):
             pass
 
     with pytest.raises(InvalidError, match="can't have parameters"):
 
         @app.function(serialized=True)
         @wsgi_app()
-        async def my_invalid_wsgi(x):
+        def my_invalid_wsgi(x):
             pass
 
     with pytest.warns(DeprecationError, match="default parameters"):
 
         @app.function(serialized=True)
         @asgi_app()
-        async def my_deprecated_default_params_asgi(x=1):
+        def my_deprecated_default_params_asgi(x=1):
             pass
 
     with pytest.warns(DeprecationError, match="default parameters"):
 
         @app.function(serialized=True)
         @wsgi_app()
-        async def my_deprecated_default_params_wsgi(x=1):
+        def my_deprecated_default_params_wsgi(x=1):
+            pass
+
+    with pytest.raises(InvalidError, match="async function"):
+
+        @app.function(serialized=True)
+        @asgi_app()
+        async def my_async_asgi_function():
+            pass
+
+    with pytest.raises(InvalidError, match="async function"):
+
+        @app.function(serialized=True)
+        @wsgi_app()
+        async def my_async_wsgi_function():
             pass
 
     async with app.run(client=client):