diff --git a/smb.h b/smb.h
index 88eaa06..b521617 100644
--- a/smb.h
+++ b/smb.h
@@ -116,7 +116,7 @@
 #define TRANSACT2_FINDNOTIFYNEXT  12
 #define TRANSACT2_MKDIR           13
 
-#define PTR_DIFF(p1, p2) ((size_t)(((char *)(p1)) - (char *)(p2)))
+#define PTR_DIFF(p1, p2) ((size_t)(((const char *)(p1)) - (const char *)(p2)))
 
 /* some protos */
 const u_char *smb_fdata(netdissect_options *, const u_char *, const char *, const u_char *, int);