diff --git a/sentry_sdk/serializer.py b/sentry_sdk/serializer.py
index df6a9053c1..134528cd9a 100644
--- a/sentry_sdk/serializer.py
+++ b/sentry_sdk/serializer.py
@@ -273,6 +273,8 @@ def _serialize_node_impl(
                 if result is not NotImplemented:
                     return _flatten_annotated(result)
 
+        sentry_repr = getattr(type(obj), "__sentry_repr__", None)
+
         if obj is None or isinstance(obj, (bool, number_types)):
             if should_repr_strings or (
                 isinstance(obj, float) and (math.isinf(obj) or math.isnan(obj))
@@ -281,8 +283,8 @@ def _serialize_node_impl(
             else:
                 return obj
 
-        elif callable(getattr(obj, "sentry_repr", None)):
-            return obj.sentry_repr()
+        elif callable(sentry_repr):
+            return sentry_repr(obj)
 
         elif isinstance(obj, datetime):
             return (
diff --git a/tests/test_serializer.py b/tests/test_serializer.py
index 503bc14fb2..1cc20c4b4a 100644
--- a/tests/test_serializer.py
+++ b/tests/test_serializer.py
@@ -1,5 +1,4 @@
 import sys
-
 import pytest
 
 from sentry_sdk.serializer import serialize
@@ -68,8 +67,20 @@ def test_serialize_sets(extra_normalizer):
 
 def test_serialize_custom_mapping(extra_normalizer):
     class CustomReprDict(dict):
-        def sentry_repr(self):
+        def __sentry_repr__(self):
             return "custom!"
 
     result = extra_normalizer(CustomReprDict(one=1, two=2))
     assert result == "custom!"
+
+
+def test_custom_mapping_doesnt_mess_with_mock(extra_normalizer):
+    """
+    Adding the __sentry_repr__ magic method check in the serializer
+    shouldn't mess with how mock works. This broke some stuff when we added
+    sentry_repr without the dunders.
+    """
+    mock = pytest.importorskip("unittest.mock")
+    m = mock.Mock()
+    extra_normalizer(m)
+    assert len(m.mock_calls) == 0