1 files changed, 2 insertions, 2 deletions
diff --git a/polymatrix/expression/mixins/additionexprmixin.py b/polymatrix/expression/mixins/additionexprmixin.py
index e068ba1..d274f2a 100644
--- a/polymatrix/expression/mixins/additionexprmixin.py
+++ b/polymatrix/expression/mixins/additionexprmixin.py
@@ -26,14 +26,14 @@ class AdditionExprMixin(ExpressionBaseMixin):
     #     return self.left.shape
 
     # overwrites abstract method of `ExpressionBaseMixin`
-    def apply(
+    def _apply(
         self, 
         state: ExpressionState,
     ) -> tuple[ExpressionState, PolyMatrix]:
         state, left = self.left.apply(state=state)
         state, right = self.right.apply(state=state)
 
-        assert left.shape == right.shape
+        assert left.shape == right.shape, f'{left.shape} != {right.shape}'
 
         terms = {}