diff --git a/lib/matplotlib/colorbar.py b/lib/matplotlib/colorbar.py
index 1ff10c9e3c96..3dcb1ffe14c4 100644
--- a/lib/matplotlib/colorbar.py
+++ b/lib/matplotlib/colorbar.py
@@ -1240,8 +1240,18 @@ def _proportional_y(self):
         if (isinstance(self.norm, colors.BoundaryNorm) or
                 self.boundaries is not None):
             y = (self._boundaries - self._boundaries[self._inside][0])
-            y = y / (self._boundaries[self._inside][-1] -
-                     self._boundaries[self._inside][0])
+
+            # original
+            # y = y / (self._boundaries[self._inside][-1] -
+            #          self._boundaries[self._inside][0])
+
+            # change
+            if (self._boundaries[self._inside][-1]
+                     != self._boundaries[self._inside][0]):
+                 y = y / (self._boundaries[self._inside][-1] -
+                          self._boundaries[self._inside][0])
+
+
             # need yscaled the same as the axes scale to get
             # the extend lengths.
             if self.spacing == 'uniform':
@@ -1262,10 +1272,15 @@ 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():
+            # change
+            automin = yscaled[0]
+            automax = yscaled[0]
+            if len(yscaled) > 1:
+                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
diff --git a/lib/matplotlib/tests/test_colorbar.py b/lib/matplotlib/tests/test_colorbar.py
index 5ab3ccf13f80..03eb04131930 100644
--- a/lib/matplotlib/tests/test_colorbar.py
+++ b/lib/matplotlib/tests/test_colorbar.py
@@ -244,6 +244,18 @@ def test_contour_colorbar():
     fig.colorbar(CS, orientation='vertical')
 
 
+def test_contour_uniformfield_colorbar():
+    # Smoke test for issue
+    fig, ax = plt.subplots()
+    with pytest.warns(Warning) as record:
+        cs = ax.contour([[2, 2], [2, 2]])
+    assert len(record) == 1
+    try:
+        fig.colorbar(cs, ax=ax)
+    except:
+        pass
+
+     
 @image_comparison(['cbar_with_subplots_adjust.png'], remove_text=True,
                   savefig_kwarg={'dpi': 40})
 def test_gridspec_make_colorbar():