diff --git a/util/readline.c b/util/readline.c
index 0f19674f52..e2664e48ca 100644
--- a/util/readline.c
+++ b/util/readline.c
@@ -84,7 +84,9 @@ static void readline_update(ReadLineState *rs)
 
 static void readline_insert_char(ReadLineState *rs, int ch)
 {
-    if (rs->cmd_buf_index < READLINE_CMD_BUF_SIZE) {
+    assert(rs->cmd_buf_index <= rs->cmd_buf_size);
+
+    if (rs->cmd_buf_size < READLINE_CMD_BUF_SIZE) {
         memmove(rs->cmd_buf + rs->cmd_buf_index + 1,
                 rs->cmd_buf + rs->cmd_buf_index,
                 rs->cmd_buf_size - rs->cmd_buf_index);