Index: lib/matplotlib/axes.py
===================================================================
--- lib/matplotlib/axes.py	(revision 8565)
+++ lib/matplotlib/axes.py	(working copy)
@@ -7587,7 +7587,12 @@
                 else:
                     raise ValueError("weights must be 1D or 2D")
             else:
-                w = [np.array(wi) for wi in weights]
+                try:
+                    weights[0][0]
+                except TypeError:
+                    w = [np.array(weights)]
+                else:
+                    w = [np.array(wi) for wi in weights]
 
             if len(w) != nx:
                 raise ValueError('weights should have the same shape as x')