diff --git a/lib/matplotlib/colorbar.py b/lib/matplotlib/colorbar.py
index 8cf8c0e6c1eb..ec88549dc49f 100644
--- a/lib/matplotlib/colorbar.py
+++ b/lib/matplotlib/colorbar.py
@@ -1221,8 +1221,10 @@ 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])
+            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':
@@ -1243,10 +1245,13 @@ 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():
+            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 149ed4c3d22e..428627f2ccef 100644
--- a/lib/matplotlib/tests/test_colorbar.py
+++ b/lib/matplotlib/tests/test_colorbar.py
@@ -242,6 +242,15 @@ def test_contour_colorbar():
     fig.colorbar(CS, orientation='vertical')
 
 
+def test_contour_uniformfield_colorbar():
+    # Smoke test for gh#23817
+    fig, ax = plt.subplots()
+    with pytest.warns(Warning) as record:
+        cs = ax.contour([[1, 1], [1, 1]])
+    assert len(record) == 1
+    fig.colorbar(cs, ax=ax)
+
+
 @image_comparison(['cbar_with_subplots_adjust.png'], remove_text=True,
                   savefig_kwarg={'dpi': 40})
 def test_gridspec_make_colorbar():