Moin!

I try to read a pushbutton on a Teensy 3.2 Pin 13 (the one with the internal LED....) the pushbutton shorting the Teensys pin to gnd.

I set up the Pin with

Code:
const int button6=13;
pinMode(button6, INPUT_PULLUP);
which works for all other pins i use except pin 13. I suppose its the internal LED that pulls the pin to GND? Id so, should i add an 10k to +3,3V as external PullUp resistor? Im asking because currently im away from the workshop, else id just solder one and see ;-)

Thanks in advance!