diff --git a/crypto.c b/crypto.c
index be65a440..1937681d 100644
--- a/crypto.c
+++ b/crypto.c
@@ -444,6 +444,7 @@ int dtls_ecdh_pre_master_secret(unsigned char *priv_key,
   uint32_t result_x[8];
   uint32_t result_y[8];
 
+  assert(key_size == sizeof(priv));
   if (result_len < key_size) {
     return -1;
   }