diff --git a/nitransforms/nonlinear.py b/nitransforms/nonlinear.py
index 69c19d35..e3a984b1 100644
--- a/nitransforms/nonlinear.py
+++ b/nitransforms/nonlinear.py
@@ -163,6 +163,8 @@ def map(self, x, inverse=False):
 
         if inverse is True:
             raise NotImplementedError
+
+        x = np.array(x)
         ijk = self.reference.index(x)
         indexes = np.round(ijk).astype("int")