diff --git a/crypto/stack/stack.c b/crypto/stack/stack.c
index 3e2f4d8786329f996bac473ad35620368a07980a..8ab4302c2e487a1d36a485241093bcb92b76c5d2 100644
--- a/crypto/stack/stack.c
+++ b/crypto/stack/stack.c
@@ -279,7 +279,7 @@ void sk_free(STACK *st)
 	Free(st);
 	}
 
-int sk_num(STACK *st)
+int sk_num(const STACK *st)
 {
 	if(st == NULL) return -1;
 	return st->num;
diff --git a/crypto/stack/stack.h b/crypto/stack/stack.h
index a6665f3b30f20b76beb5c7b4bc7337ab08530b9d..0cc3b44cc077ce272a467413052b9b093ae511bb 100644
--- a/crypto/stack/stack.h
+++ b/crypto/stack/stack.h
@@ -79,7 +79,7 @@ typedef struct stack_st
 #define M_sk_num(sk)		((sk) ? (sk)->num:-1)
 #define M_sk_value(sk,n)	((sk) ? (sk)->data[n] : NULL)
 
-int sk_num(STACK *);
+int sk_num(const STACK *);
 char *sk_value(STACK *, int);
 
 char *sk_set(STACK *, int, char *);