diff --git a/crypto/stack/stack.c b/crypto/stack/stack.c
index 1d2b47ee67..821b6caca5 100644
--- a/crypto/stack/stack.c
+++ b/crypto/stack/stack.c
@@ -307,7 +307,7 @@ int sk_num(const STACK *st)
 
 char *sk_value(const STACK *st, int i)
 {
-	if(st == NULL) return NULL;
+	if(!st || (i < 0) || (i >= st->num)) return NULL;
 	return st->data[i];
 }