1 files changed, 7 insertions, 2 deletions
diff --git a/ext/kconv/kconv.c b/ext/kconv/kconv.c
index d9bb2b4d3d..a3349826f1 100644
--- a/ext/kconv/kconv.c
+++ b/ext/kconv/kconv.c
@@ -1891,10 +1891,15 @@ static VALUE
 kconv_guess(obj, src)
     VALUE obj, src;
 {
-    unsigned char *p = RSTRING(src)->ptr;
-    unsigned char *pend = p + RSTRING(src)->len;
+    unsigned char *p;
+    unsigned char *pend;
     int sequence_counter = 0;
 
+    Check_Type(src, T_STRING);
+
+    p = RSTRING(src)->ptr;
+    pend = p + RSTRING(src)->len;
+
 #define INCR do {\
     p++;\
     if (p==pend) return INT2FIX(_UNKNOWN);\