diff --git a/src/if_perl.xs b/src/if_perl.xs
index 2655640..099d386 100644
--- a/src/if_perl.xs
+++ b/src/if_perl.xs
@@ -1570,7 +1570,7 @@
     newsv = newSVpv("0z", 2);
     for (i = 0; i < len; i++)
     {
-	sprintf(buf, "%02X", s[i]);
+	sprintf(buf, "%02X", (unsigned char)(s[i]));
 	sv_catpvn(newsv, buf, 2);
     }
     RETVAL = newsv;