VBAT ADC voltage measurement incorrect at low voltages

Hello,

I have a problem with VBAT voltage measurement on a STM32F401VB.

As can be seen in the picture below, the ADC output is correct (identical to the calculated value ''ADC Should'') in the upper region of the input voltage, but incorrect when the input voltage drops below approximately 1V. 

At 0V input voltage the ADC value is 239 when it should be close to zero.

The other ADC channels work as expected, so I think this has something to do with the internal VBAT circuitry.

Does anybody know what this could be caused by?

Thanks and Kind Regards

Daniel