diff --git a/src/vectise/colours.py b/src/vectise/colours.py
index b8bdd53..9657239 100644
--- a/src/vectise/colours.py
+++ b/src/vectise/colours.py
@@ -161,7 +161,7 @@ class BinaryColourMap(ColourMap):
         self.colours = colours
 
     def __call__(self, v: float):
-        return self.colours[np.isclose(v, 0)]
+        return self.colours[np.isclose(v, 0).astype(int)]
 
 
 class RandomChoiceColourMap(ColourMap):