diff --git a/crypto/bn_extra/convert.c b/crypto/bn_extra/convert.c
index a45943a737..7cc665cc10 100644
--- a/crypto/bn_extra/convert.c
+++ b/crypto/bn_extra/convert.c
@@ -452,7 +452,7 @@ BIGNUM *BN_mpi2bn(const uint8_t *in, size_t len, BIGNUM *out) {
     if (num_bits >= INT_MAX) {
       return NULL;
     }
-    BN_clear_bit(out, (int)BN_num_bits(out) - 1);
+    BN_clear_bit(out, (int)num_bits - 1);
   }
   return out;
 }