diff --git a/src/gfx.cpp b/src/gfx.cpp
--- a/src/gfx.cpp
+++ b/src/gfx.cpp
@@ -74,10 +74,9 @@ static Rect _invalid_rect;
 static const byte *_colour_remap_ptr;
 static byte _string_colourremap[3]; ///< Recoloursprite for stringdrawing. The grf loader ensures, that ST_FONT sprites only use colours 0 to 2.
 
-enum {
-	DIRTY_BLOCK_HEIGHT   = 8,
-	DIRTY_BLOCK_WIDTH    = 64,
-};
+static const uint DIRTY_BLOCK_HEIGHT   = 8;
+static const uint DIRTY_BLOCK_WIDTH    = 64;
+
 static uint _dirty_bytes_per_line = 0;
 static byte *_dirty_blocks = NULL;