diff --git a/crypto/modes/ctr.c b/crypto/modes/ctr.c
index a5a35899..306b6f74 100644
--- a/crypto/modes/ctr.c
+++ b/crypto/modes/ctr.c
@@ -163,7 +163,8 @@ void CRYPTO_ctr128_encrypt_ctr32(const uint8_t *in, uint8_t *out,
                                  unsigned int *num, ctr128_f func) {
   unsigned int n, ctr32;
 
-  assert(in && out && key && ecount_buf && num);
+  assert(key && ecount_buf && num);
+  assert(len == 0 || (in && out));
   assert(*num < 16);
 
   n = *num;