Mysqli :: commit（）返回false，但没有错误消息

I have three sql queries in a try/catch block. Autocommit is off and all three queries run without errors, I have verified that (it shouldn't reach the commit line if any query fails, but I've verified it via xdebug, stepping through the code one line at a time). When the code reached the commit() function call, mysqli returns false, but no error message. How can a commit fail when the previous queries all worked?I have three sql queries in a try/catch block.