diff --git a/lib/matplotlib/colorbar.py b/lib/matplotlib/colorbar.py
index 5894ecf123c2..7ed4a9877498 100644
--- a/lib/matplotlib/colorbar.py
+++ b/lib/matplotlib/colorbar.py
@@ -801,6 +801,7 @@ def _proportional_y(self):
             y = y / (self._boundaries[-1] - self._boundaries[0])
         else:
             y = self.norm(self._boundaries.copy())
+            y = np.ma.filled(y, np.nan)
         if self.extend == 'min':
             # Exclude leftmost interval of y.
             clen = y[-1] - y[1]
@@ -811,21 +812,22 @@ def _proportional_y(self):
             clen = y[-2] - y[0]
             automin = (y[1] - y[0]) / clen
             automax = (y[-2] - y[-3]) / clen
-        else:
+        elif self.extend == 'both':
             # Exclude leftmost and rightmost intervals in y.
             clen = y[-2] - y[1]
             automin = (y[2] - y[1]) / clen
             automax = (y[-2] - y[-3]) / clen
-        extendlength = self._get_extension_lengths(self.extendfrac,
-                                                   automin, automax,
-                                                   default=0.05)
+        if self.extend in ('both', 'min', 'max'):
+            extendlength = self._get_extension_lengths(self.extendfrac,
+                                                       automin, automax,
+                                                       default=0.05)
         if self.extend in ('both', 'min'):
             y[0] = 0. - extendlength[0]
         if self.extend in ('both', 'max'):
             y[-1] = 1. + extendlength[1]
         yi = y[self._inside]
         norm = colors.Normalize(yi[0], yi[-1])
-        y[self._inside] = norm(yi)
+        y[self._inside] = np.ma.filled(norm(yi), np.nan)
         return y
 
     def _mesh(self):