diff --git a/src/if_ruby.c b/src/if_ruby.c
index d3ed781..543ee8a 100644
--- a/src/if_ruby.c
+++ b/src/if_ruby.c
@@ -1267,7 +1267,7 @@
     int	i;
     for (i = 0; i < RSTRING_LEN(str); i++)
     {
-	sprintf(buf, "%02X", RSTRING_PTR(str)[i]);
+	sprintf(buf, "%02X", (unsigned char)(RSTRING_PTR(str)[i]));
 	rb_str_concat(result, rb_str_new2(buf));
     }
     return result;