diff --git a/tests/test_descriptor.py b/tests/test_descriptor.py
index c6173a6..5a0a43e 100644
--- a/tests/test_descriptor.py
+++ b/tests/test_descriptor.py
@@ -184,8 +184,8 @@ def test_descriptor_metadata():
 class WithSetAttr:
     """Modify the value before saving it."""
 
-    value = Descriptor(on_setattr=lambda self, value: value + 1)
-    value_frozen = Descriptor(frozen=True, on_setattr=lambda self, value: value + 1)
+    value = Descriptor(on_setattr=lambda value: value + 1)
+    value_frozen = Descriptor(frozen=True, on_setattr=lambda value: value + 1)
 
 
 def test_with_setattr():
diff --git a/zninit/descriptor/__init__.py b/zninit/descriptor/__init__.py
index fb94e14..c372d7c 100644
--- a/zninit/descriptor/__init__.py
+++ b/zninit/descriptor/__init__.py
@@ -170,7 +170,7 @@ def __set__(self, instance, value):
         if self._frozen.get(instance, False):
             raise TypeError(f"Frozen attribute '{self.name}' can not be changed.")
         if self.on_setattr is not None:
-            value = self.on_setattr(instance, value)
+            value = self.on_setattr(value)
         if self.check_types:
             typeguard.check_type(
                 argname=self.name, value=value, expected_type=self.annotation