diff --git a/ninja/signature/details.py b/ninja/signature/details.py
index a05903c9..cb79cf1e 100644
--- a/ninja/signature/details.py
+++ b/ninja/signature/details.py
@@ -72,6 +72,15 @@ def __init__(self, path: str, view_func: Callable[..., Any]) -> None:
                 self.response_arg = name
                 continue
 
+            if (
+                arg.annotation is inspect.Parameter.empty
+                and isinstance(arg.default, type)
+                and issubclass(arg.default, pydantic.BaseModel)
+            ):
+                raise ConfigError(
+                    f"Looks like you are using `{name}={arg.default.__name__}` instead of `{name}: {arg.default.__name__}` (annotation)"
+                )
+
             func_param = self._get_param_type(name, arg)
             self.params.append(func_param)
 
diff --git a/tests/test_body.py b/tests/test_body.py
index 0f288460..bbb1d4a7 100644
--- a/tests/test_body.py
+++ b/tests/test_body.py
@@ -1,6 +1,8 @@
+import pytest
 from pydantic import field_validator
 
 from ninja import Body, Form, NinjaAPI, Schema
+from ninja.errors import ConfigError
 from ninja.testing import TestClient
 
 api = NinjaAPI()
@@ -66,3 +68,17 @@ def test_body_validation_error():
             "ctx": {"error": "invalid email"},
         }
     ]
+
+
+def test_incorrect_annotation():
+    api = NinjaAPI()
+
+    class Some(Schema):
+        a: int
+
+    with pytest.raises(ConfigError):
+
+        @api.post("/some")
+        def some(request, payload=Some):
+            #  ................. ^------ invalid usage assigning class instead of annotation
+            return 42