diff --git a/lib/vtls/wolfssl.c b/lib/vtls/wolfssl.c
index 693cbdc92e..0cf6e0e4a5 100644
--- a/lib/vtls/wolfssl.c
+++ b/lib/vtls/wolfssl.c
@@ -362,8 +362,11 @@ static int wssl_bio_cf_in_read(WOLFSSL_BIO *bio, char *buf, int blen)
   CURLcode result = CURLE_OK;
 
   DEBUGASSERT(data);
-  /* OpenSSL catches this case, so should we. */
-  if(!buf)
+  if(!data || (blen < 0)) {
+    wssl->io_result = CURLE_FAILED_INIT;
+    return -1;
+  }
+  if(!buf || !blen)
     return 0;
 
   if((connssl->connecting_state == ssl_connect_2) &&