DFPlayerMini: What does "WrongStack" mean and what causes it?

I've got a NODE MCU and am using the Arduino IDE. My project is a talking clock. I create an array of file names and then play them to speak the time. I've got an intermittent problem where readType() returns 1 and then the files stop playing. Of course, this isn't optimal when one is making an alarm clock. Does anyone have any suggestions? I'm using the DFRobot library.