diff --git a/scrapy/core/spidermw.py b/scrapy/core/spidermw.py
index 230332673..b24ccf6ae 100644
--- a/scrapy/core/spidermw.py
+++ b/scrapy/core/spidermw.py
@@ -20,10 +20,6 @@ def _isiterable(possible_iterator):
     return hasattr(possible_iterator, '__iter__') or hasattr(possible_iterator, '__aiter__')
 
 
-def _fname(f):
-    return f"{f.__self__.__class__.__name__}.{f.__func__.__name__}"
-
-
 class SpiderMiddlewareManager(MiddlewareManager):
 
     component_name = 'spider middleware'
@@ -48,7 +44,7 @@ class SpiderMiddlewareManager(MiddlewareManager):
             try:
                 result = method(response=response, spider=spider)
                 if result is not None:
-                    msg = (f"Middleware {_fname(method)} must return None "
+                    msg = (f"Middleware {method.__qualname__} must return None "
                            f"or raise an exception, got {type(result)}")
                     raise _InvalidOutput(msg)
             except _InvalidOutput:
@@ -88,7 +84,7 @@ class SpiderMiddlewareManager(MiddlewareManager):
             elif result is None:
                 continue
             else:
-                msg = (f"Middleware {_fname(method)} must return None "
+                msg = (f"Middleware {method.__qualname__} must return None "
                        f"or an iterable, got {type(result)}")
                 raise _InvalidOutput(msg)
         return _failure
@@ -96,7 +92,8 @@ class SpiderMiddlewareManager(MiddlewareManager):
     def _process_spider_output(self, response, spider, result, start_index=0):
         # items in this iterable do not need to go through the process_spider_output
         # chain, they went through it already from the process_spider_exception method
-        if isinstance(result, collections.abc.AsyncIterator):
+        result_async = isinstance(result, collections.abc.AsyncIterator)
+        if result_async:
             recovered = MutableAsyncChain()
         else:
             recovered = MutableChain()
@@ -116,9 +113,11 @@ class SpiderMiddlewareManager(MiddlewareManager):
             if _isiterable(result):
                 result = self._evaluate_iterable(response, spider, result, method_index + 1, recovered)
             else:
-                msg = (f"Middleware {_fname(method)} must return an "
+                msg = (f"Middleware {method.__qualname__} must return an "
                        f"iterable, got {type(result)}")
                 raise _InvalidOutput(msg)
+            if result_async and isinstance(result, collections.abc.Iterator):
+                raise TypeError(f"Synchronous {method.__qualname__} called with an async iterable")
 
         # check this again as the middlewares could change "result" from sync to async
         if isinstance(result, collections.abc.AsyncIterator):
diff --git a/tests/test_spidermiddleware.py b/tests/test_spidermiddleware.py
index 2584dec21..0a6b96c0c 100644
--- a/tests/test_spidermiddleware.py
+++ b/tests/test_spidermiddleware.py
@@ -122,11 +122,6 @@ class BaseAsyncSpiderMiddlewareTestCase(SpiderMiddlewareTestCase):
         result = yield self.mwman.scrape_response(self._scrape_func, self.response, self.request, self.spider)
         return result
 
-    def assertAsyncGeneratorNotIterable(self, o):
-        with self.assertRaisesRegex(TypeError,
-                                    "'(async_generator|MutableAsyncChain)' object is not iterable"):
-            list(o)
-
     @defer.inlineCallbacks
     def _test_simple_base(self, *mw_classes):
         result = yield self._get_middleware_result(*mw_classes)
@@ -145,9 +140,8 @@ class BaseAsyncSpiderMiddlewareTestCase(SpiderMiddlewareTestCase):
 
     @defer.inlineCallbacks
     def _test_asyncgen_fail(self, *mw_classes):
-        result = yield self._get_middleware_result(*mw_classes)
-        self.assertIsInstance(result, collections.abc.Iterable)
-        self.assertAsyncGeneratorNotIterable(result)
+        with self.assertRaisesRegex(TypeError, "Synchronous .+ called with an async iterable"):
+            yield self._get_middleware_result(*mw_classes)
 
 
 class ProcessSpiderOutputSimpleMiddleware:
@@ -238,14 +232,10 @@ class ProcessSpiderOutputAsyncGen(ProcessSpiderOutputSimple):
         """ Simple mw; cannot work """
         return self._test_asyncgen_fail(self.MW_SIMPLE)
 
-    @defer.inlineCallbacks
     def test_simple_asyncgen(self):
         """ Simple mw -> asyncgen mw; cannot work """
-        result = yield self._get_middleware_result(
-            self.MW_ASYNCGEN,
-            self.MW_SIMPLE)
-        self.assertIsInstance(result, collections.abc.AsyncIterable)
-        self.assertAsyncGeneratorNotIterable(result)
+        return self._test_asyncgen_fail(self.MW_ASYNCGEN,
+                                        self.MW_SIMPLE)
 
     def test_universal(self):
         """ Universal mw """