diff --git a/src/direction_type.h b/src/direction_type.h
index 55e430e8cd..f025d113e7 100644
--- a/src/direction_type.h
+++ b/src/direction_type.h
@@ -120,7 +120,7 @@ DECLARE_POSTFIX_INCREMENT(DiagDirDiff)
  * (and south-east edge). The Y axis must be so the one which goes
  * align the north-east edge (and south-west) edge.
  */
-enum Axis {
+enum Axis : byte {
 	AXIS_X = 0,          ///< The X axis
 	AXIS_Y = 1,          ///< The y axis
 	AXIS_END,            ///< Used for iterations