diff --git a/src/hci/readline.c b/src/hci/readline.c
index 5b46413..3c5f924 100644
--- a/src/hci/readline.c
+++ b/src/hci/readline.c
@@ -278,6 +278,7 @@
 	/* Prefill string */
 	if ( ( rc = replace_string ( &string, prefill ) ) != 0 )
 		goto error;
+	assert ( *line != NULL );
 	sync_console ( &string );
 
 	while ( 1 ) {