diff --git a/lib/transfer.c b/lib/transfer.c
index 0301167dc51d87abda0e42e89d2d9c59fa5da905..56cc46cfab057c6debd2f19b91b3d97dda48f984 100644
--- a/lib/transfer.c
+++ b/lib/transfer.c
@@ -1951,6 +1951,8 @@ Curl_Transfer(struct connectdata *c_conn, /* connection data */
   if(!conn)
     return CURLE_BAD_FUNCTION_ARGUMENT;
 
+  curlassert(sockindex <= 1);
+
   /* now copy all input parameters */
   conn->sockfd = sockindex==-1?-1:conn->sock[sockindex];
   conn->size = size;