diff --git a/src/map/IMG/subfile.h b/src/map/IMG/subfile.h
index bc2fb723..167fee74 100644
--- a/src/map/IMG/subfile.h
+++ b/src/map/IMG/subfile.h
@@ -5,8 +5,7 @@
 #include <QFile>
 #include "imgdata.h"
 
-
-#define BLOCK_BITS 12 /* 4096 bytes */
+#define BLOCK_BITS 9 /* 512 bytes */
 
 namespace IMG {