diff --git a/ext/date/date_parse.c b/ext/date/date_parse.c
index 1214f39..d7d22fa 100644
--- a/ext/date/date_parse.c
+++ b/ext/date/date_parse.c
@@ -441,8 +441,10 @@ date_zone_to_diff(VALUE str)
 	    char *s, *p;
 	    VALUE sign;
 	    VALUE hour = Qnil, min = Qnil, sec = Qnil;
+	    volatile VALUE str_orig;
 
 	    s = RSTRING_PTR(str);
+	    str_orig = str;
 
 	    if (strncmp(s, "gmt", 3) == 0 ||
 		strncmp(s, "utc", 3) == 0)