diff --git a/nitransforms/nonlinear.py b/nitransforms/nonlinear.py
index ef92eb6c..c0cdc92e 100644
--- a/nitransforms/nonlinear.py
+++ b/nitransforms/nonlinear.py
@@ -165,7 +165,7 @@ def map(self, x, inverse=False):
             indexes = tuple(tuple(i) for i in indexes.T)
             return self._field[indexes]
 
-        return np.vstack((
+        return np.vstack(tuple(
             map_coordinates(
                 self._field[..., i],
                 ijk.T,