diff --git a/kwant/plotter.py b/kwant/plotter.py
index 2ce80731f81b473379cbdf500b6298e6f74b50f2..ae4371cc1e618cccf9eb4c556aaeed689f48decf 100644
--- a/kwant/plotter.py
+++ b/kwant/plotter.py
@@ -757,7 +757,7 @@ def mask_interpolate(coords, values, a=None, method='nearest', oversampling=3):
     elif a <= 0:
         raise ValueError("The distance a must be strictly positive.")
 
-    shape = (((cmin - cmax) / a + 1) * oversampling).round()
+    shape = (((cmax - cmin) / a + 1) * oversampling).round()
     delta = 0.5 * (oversampling - 1) * a / oversampling
     cmin -= delta
     cmax += delta