diff --git a/xarray/plot/utils.py b/xarray/plot/utils.py
index a5fb27350db..707a3c5f8da 100644
--- a/xarray/plot/utils.py
+++ b/xarray/plot/utils.py
@@ -215,14 +215,16 @@ def _determine_cmap_params(plot_data, vmin=None, vmax=None, cmap=None,
             norm.vmin = vmin
         else:
             if not vmin_was_none and vmin != norm.vmin:
-                raise ValueError('Cannot supply vmin and a norm with vmin.')
+                raise ValueError('Cannot supply vmin and a norm'
+                                 + ' with a different vmin.')
             vmin = norm.vmin
 
         if norm.vmax is None:
             norm.vmax = vmax
         else:
             if not vmax_was_none and vmax != norm.vmax:
-                raise ValueError('Cannot supply vmax and a norm with vmax.')
+                raise ValueError('Cannot supply vmax and a norm'
+                                 + ' with a different vmax.')
             vmax = norm.vmax
 
     # if BoundaryNorm, then set levels