diff --git a/lib/matplotlib/colorbar.py b/lib/matplotlib/colorbar.py
index c8f49c0ad92d..7f1930484283 100644
--- a/lib/matplotlib/colorbar.py
+++ b/lib/matplotlib/colorbar.py
@@ -1243,10 +1243,14 @@ def _proportional_y(self):
         yscaled = np.ma.filled(norm(yscaled), np.nan)
         # make the lower and upper extend lengths proportional to the lengths
         # of the first and last boundary spacing (if extendfrac='auto'):
-        automin = yscaled[1] - yscaled[0]
-        automax = yscaled[-1] - yscaled[-2]
         extendlength = [0, 0]
         if self._extend_lower() or self._extend_upper():
+            if len(yscaled) == 1:
+                automin = yscaled[0]
+                automax = yscaled[0]
+            else:
+                automin = yscaled[1] - yscaled[0]
+                automax = yscaled[-1] - yscaled[-2]
             extendlength = self._get_extension_lengths(
                     self.extendfrac, automin, automax, default=0.05)
         return y, extendlength