diff --git a/lib/libX11/src/ConnDis.c b/lib/libX11/src/ConnDis.c
index 2f0cd496e..8b8639932 100644
--- a/lib/libX11/src/ConnDis.c
+++ b/lib/libX11/src/ConnDis.c
@@ -1131,7 +1131,7 @@ GetAuthorization(
 	    
 	    /* In the case of v4 mapped addresses send the v4 
 	       part of the address - addr is already in network byte order */
-	    if (memcmp(addr+8, ipv4mappedprefix, 12) == 0) {
+	    if (memcmp((char *)addr+8, ipv4mappedprefix, 12) == 0) {
 		for (i = 20 ; i < 24; i++)
 		    xdmcp_data[j++] = ((char *)addr)[i];