Changing walktime in XMC1202 on the Arduino shield to low values

Hello, I have one question regarding the Infineon Arduino Shield shield with an XMC1202, KITLEDXMC1202AS01TOBO1

Does anyone know why it is not possible to set WALKTIME lower than 15?

This code:

I2CWRITE2BYTES (ADDRESS, WALKTIME, 25);
works fine, while this code:

I2CWRITE2BYTES (ADDRESS, WALKTIME, 5)
sets the walktime to 15.

I suppose there is some coding in the XMC1202 itself?
I need to switch off the walktime entirely, so any help would be appreciated very much.

Thanks a lot!