diff --git a/include/bout/mask.hxx b/include/bout/mask.hxx
index 4250d21105..fd90ae7345 100644
--- a/include/bout/mask.hxx
+++ b/include/bout/mask.hxx
@@ -66,6 +66,8 @@ public:
 
   inline bool& operator()(int jx, int jy, int jz) { return mask(jx, jy, jz); }
   inline const bool& operator()(int jx, int jy, int jz) const { return mask(jx, jy, jz); }
+
+  inline bool& operator[](const Ind3D& i) { return mask[i]; }
   inline const bool& operator[](const Ind3D& i) const { return mask[i]; }
 };