diff --git a/src/r_defs.h b/src/r_defs.h
index 02da2ac0ee..6a3dfac762 100644
--- a/src/r_defs.h
+++ b/src/r_defs.h
@@ -394,7 +394,7 @@ public:
 	}
 
 	// Moves a plane up/down by hdiff units
-	double GetChangedHeight(double hdiff)
+	double GetChangedHeight(double hdiff) const
 	{
 		return D - hdiff * normal.Z;
 	}
diff --git a/wadsrc/static/zscript/mapdata.txt b/wadsrc/static/zscript/mapdata.txt
index 8914fd4056..99134a70a6 100644
--- a/wadsrc/static/zscript/mapdata.txt
+++ b/wadsrc/static/zscript/mapdata.txt
@@ -184,7 +184,7 @@ struct SecPlane native play
 	native bool isEqual(Secplane other) const;
 	native void ChangeHeight(double hdiff);
 	native double GetChangedHeight(double hdiff) const;
-	native double HeightDiff(double oldd, double newd = 0.0);
+	native double HeightDiff(double oldd, double newd = 0.0) const;
 	native double PointToDist(Vector2 xy, double z) const;
 }