diff --git a/xarray/plot/plot.py b/xarray/plot/plot.py
index 79c4cf7cb6e..8699007ca81 100644
--- a/xarray/plot/plot.py
+++ b/xarray/plot/plot.py
@@ -445,8 +445,9 @@ def newplotfunc(darray, x=None, y=None, figsize=None, size=None,
         # Decide on a default for the colorbar before facetgrids
         if add_colorbar is None:
             add_colorbar = plotfunc.__name__ != 'contour'
-        imshow_rgb = plotfunc.__name__ == 'imshow' and \
-            darray.ndim == (3 + (row is not None) + (col is not None))
+        imshow_rgb = (
+            plotfunc.__name__ == 'imshow' and
+            darray.ndim == (3 + (row is not None) + (col is not None)))
         if imshow_rgb:
             # Don't add a colorbar when showing an image with explicit colors
             add_colorbar = False
@@ -696,7 +697,8 @@ def imshow(x, y, z, ax, **kwargs):
         # there isn't one, and set it to transparent where data is masked.
         if z.shape[-1] == 3:
             z = np.ma.concatenate((z, np.ma.ones(z.shape[:2] + (1,))), 2)
-        z[np.sum(z.mask, axis=-1, dtype=bool),-1] = 0
+        z = z.copy()
+        z[np.any(z.mask, axis=-1), -1] = 0
 
     primitive = ax.imshow(z, **defaults)
 
diff --git a/xarray/plot/utils.py b/xarray/plot/utils.py
index 0de94fd12a7..e476522f587 100644
--- a/xarray/plot/utils.py
+++ b/xarray/plot/utils.py
@@ -267,7 +267,9 @@ def _infer_xy_labels_3d(darray, x, y, rgb):
     assert darray.ndim == 3
     not_none = [a for a in (x, y, rgb) if a is not None]
     if len(set(not_none)) < len(not_none):
-        raise ValueError('Dimensions passed as x, y, and rgb must be unique.')
+        raise ValueError(
+            'Dimension names must be None or unique strings, but imshow was '
+            'passed x=%r, y=%r, and rgb=%r.' % (x, y, rgb))
     for label in not_none:
         if label not in darray.dims:
             raise ValueError('%r is not a dimension' % (label,))