diff --git a/src/accounting/utils/cast.py b/src/accounting/utils/cast.py
index 0908b65..86a0e46 100644
--- a/src/accounting/utils/cast.py
+++ b/src/accounting/utils/cast.py
@@ -31,6 +31,7 @@ def be(expression: t.Any) -> sa.BinaryExpression:
     :param expression: The binary expression.
     :return: The binary expression itself.
     """
+    assert isinstance(expression, sa.BinaryExpression)
     return expression