diff --git a/ndcube/ndcube.py b/ndcube/ndcube.py
index f698634ba..445912c65 100644
--- a/ndcube/ndcube.py
+++ b/ndcube/ndcube.py
@@ -503,7 +503,7 @@ def crop_by_extra_coord(self, coord_name, min_coord_value, max_coord_value):
         result: `ndcube.NDCube`
 
         """
-        if isinstance(coord_name, str):
+        if not isinstance(coord_name, str):
             raise TypeError("The API for this function has changed. "
                             "Please give coord_name, min_coord_value, max_coord_value")
         extra_coord_dict = self.extra_coords[coord_name]