Wow, I can’t say how much time I wasted with this bug. It was crazy because the serial monitor worked but instead of my time (reading an external RTC over I2C) I got only shit, and I thought I have wrong variable types. But only after few days I make an fully restart (my notebook usually go to standby) there was suddenly a spark core arduino mouse which jumped funny around and now I find your bug fix. 
This given me a lot of gray hairs, I promise. 
greetings from Austria!