diff --git a/crypto/bn/bn_lib.c b/crypto/bn/bn_lib.c
index 9c2c9d3238a4b682c1e73eab6f0788a658fa8e9a..5d62d88e8b603ed4ad06c4becbfb45e7e0a8d8c2 100644
--- a/crypto/bn/bn_lib.c
+++ b/crypto/bn/bn_lib.c
@@ -489,6 +489,8 @@ BIGNUM *BN_dup(const BIGNUM *a)
 	{
 	BIGNUM *r;
 
+	if (a == NULL) return NULL;
+
 	bn_check_top(a);
 
 	r=BN_new();