--- OLD/xdiff/xmerge.c	Thu Jan  1 00:00:00 1970
+++ NEW/xdiff/xmerge.c	Thu Jan  1 00:00:00 1970
@@ -22,6 +22,8 @@
 
 #include "xinclude.h"
 
+#define xisalnum(x) isalnum((unsigned char)(x))
+
 typedef struct s_xdmerge {
 	struct s_xdmerge *next;
 	/*
@@ -314,7 +316,7 @@
 static int line_contains_alnum(const char *ptr, long size)
 {
 	while (size--)
-		if (isalnum(*(ptr++)))
+		if (xisalnum(*(ptr++)))
 			return 1;
 	return 0;
 }