diff --git a/nitransforms/nonlinear.py b/nitransforms/nonlinear.py
index 0aa6d36b..f4b95142 100644
--- a/nitransforms/nonlinear.py
+++ b/nitransforms/nonlinear.py
@@ -166,7 +166,7 @@ def map(self, x, inverse=False):
             indexes = tuple(tuple(i) for i in indexes)
             return self._field[indexes]
 
-        return np.vstack(
+        new_map = np.vstack(
             tuple(
                 map_coordinates(
                     self._field[..., i],