diff --git a/fastapi/dependencies/utils.py b/fastapi/dependencies/utils.py
index 97c1f7eaf..3d969ed10 100644
--- a/fastapi/dependencies/utils.py
+++ b/fastapi/dependencies/utils.py
@@ -251,11 +251,11 @@ def is_scalar_sequence_field(field: ModelField) -> bool:
 
 
 def is_scalar_mapping_field(field: ModelField) -> bool:
-    if (field.shape in mapping_shapes) and not lenient_issubclass(
-        field.type_, BaseModel
+    if (
+        (field.shape in mapping_shapes)
+        and not lenient_issubclass(field.type_, BaseModel)
+        and field.sub_fields is not None
     ):
-        if field.sub_fields is None:
-            return False
         for sub_field in field.sub_fields:
             if not is_scalar_field(sub_field):
                 return False
@@ -264,11 +264,11 @@ def is_scalar_mapping_field(field: ModelField) -> bool:
 
 
 def is_scalar_sequence_mapping_field(field: ModelField) -> bool:
-    if (field.shape in mapping_shapes) and not lenient_issubclass(
-        field.type_, BaseModel
+    if (
+        (field.shape in mapping_shapes)
+        and not lenient_issubclass(field.type_, BaseModel)
+        and field.sub_fields is not None
     ):
-        if field.sub_fields is None:
-            return False
         for sub_field in field.sub_fields:
             if not is_scalar_sequence_field(sub_field):
                 return False
diff --git a/tests/test_invalid_mapping_param.py b/tests/test_invalid_mapping_param.py
new file mode 100644
index 000000000..ef49a3218
--- /dev/null
+++ b/tests/test_invalid_mapping_param.py
@@ -0,0 +1,15 @@
+from typing import Mapping, List
+
+import pytest
+from fastapi import FastAPI, Query
+from pydantic import BaseModel
+
+
+def test_invalid_sequence():
+    with pytest.raises(AssertionError):
+        app = FastAPI()
+
+
+        @app.get("/items/")
+        def read_items(q: Mapping[str, List[List[str]]] = Query(default=None)):
+            pass  # pragma: no cover