diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 12e1c1b..5ea8afa 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -3243,6 +3243,10 @@
     if( ret != 0 )
         goto cleanup;
 
+    /* If ssl->conf->endpoint is not one of MBEDTLS_SSL_IS_CLIENT or
+     * MBEDTLS_SSL_IS_SERVER, this is the return code we give */
+    ret = MBEDTLS_ERR_SSL_BAD_INPUT_DATA;
+
 #if defined(MBEDTLS_SSL_CLI_C)
     if( ssl->conf->endpoint == MBEDTLS_SSL_IS_CLIENT )
     {