1 files changed, 3 insertions, 1 deletions
diff --git a/error.c b/error.c
index 0b98584c54..febaad2c87 100644
--- a/error.c
+++ b/error.c
@@ -907,8 +907,10 @@ syserr_initialize(argc, argv, self)
     else err = "unknown error";
     if (!NIL_P(mesg)) {
 	VALUE str = mesg;
-	size_t len = strlen(err)+RSTRING(str)->len+3;
+	size_t len;
+
 	StringValue(str);
+	len = strlen(err)+RSTRING(str)->len+3;
 	mesg = rb_str_new(0, len);
 	snprintf(RSTRING(mesg)->ptr, len+1, "%s - %.*s", err,
 		(int)RSTRING(str)->len, RSTRING(str)->ptr);