diff --git a/lib/matplotlib/colorbar.py b/lib/matplotlib/colorbar.py
index 9d40ac8e5e9c..d826649af167 100644
--- a/lib/matplotlib/colorbar.py
+++ b/lib/matplotlib/colorbar.py
@@ -1198,8 +1198,9 @@ def _proportional_y(self):
         a proportional colorbar, plus extension lengths if required:
         """
         if isinstance(self.norm, colors.BoundaryNorm):
-            y = (self._boundaries - self._boundaries[0])
-            y = y / (self._boundaries[-1] - self._boundaries[0])
+            y = (self._boundaries - 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':
diff --git a/lib/matplotlib/tests/baseline_images/test_colorbar/proportional_colorbars.png b/lib/matplotlib/tests/baseline_images/test_colorbar/proportional_colorbars.png
new file mode 100644
index 000000000000..a1f9745230ba
Binary files /dev/null and b/lib/matplotlib/tests/baseline_images/test_colorbar/proportional_colorbars.png differ
diff --git a/lib/matplotlib/tests/test_colorbar.py b/lib/matplotlib/tests/test_colorbar.py
index 5f19a0aaf20a..00a6f52698e2 100644
--- a/lib/matplotlib/tests/test_colorbar.py
+++ b/lib/matplotlib/tests/test_colorbar.py
@@ -827,3 +827,30 @@ def test_aspects():
     np.testing.assert_almost_equal(
         cb[1][0].ax.get_position(original=False).height * 2,
         cb[1][2].ax.get_position(original=False).height, decimal=2)
+
+
+@image_comparison(['proportional_colorbars.png'], remove_text=True,
+                  style='mpl20')
+def test_proportional_colorbars():
+
+    x = y = np.arange(-3.0, 3.01, 0.025)
+    X, Y = np.meshgrid(x, y)
+    Z1 = np.exp(-X**2 - Y**2)
+    Z2 = np.exp(-(X - 1)**2 - (Y - 1)**2)
+    Z = (Z1 - Z2) * 2
+
+    levels = [-1.25, -0.5, -0.125, 0.125, 0.5, 1.25]
+    cmap = mcolors.ListedColormap(
+        ['0.3', '0.5', 'white', 'lightblue', 'steelblue'])
+    cmap.set_under('darkred')
+    cmap.set_over('crimson')
+    norm = mcolors.BoundaryNorm(levels, cmap.N)
+
+    extends = ['neither', 'both']
+    spacings = ['uniform', 'proportional']
+    fig, axs = plt.subplots(2, 2)
+    for i in range(2):
+        for j in range(2):
+            CS3 = axs[i, j].contourf(X, Y, Z, levels, cmap=cmap, norm=norm,
+                                     extend=extends[i])
+            fig.colorbar(CS3, spacing=spacings[j], ax=axs[i, j])