diff --git a/source/BlockID.cpp b/source/BlockID.cpp
index b1d00d391..81be6df96 100644
--- a/source/BlockID.cpp
+++ b/source/BlockID.cpp
@@ -29,7 +29,7 @@ class cBlockIDMap
 	// Making the map case-insensitive:
 	struct Comparator
 	{
-		bool operator()(const AString & a_Item1, const AString & a_Item2)
+		bool operator()(const AString & a_Item1, const AString & a_Item2) const
 		{
 			return (NoCaseCompare(a_Item1, a_Item2) > 0);
 		}