Cortex-M0 Hardfault Handler does not get called after changing it

Question

i am using an using an STM32f0 and getting into Hardfault. My IDE is Keil MDK. I was trying to find the roots of it, so every approach i find in the web (mostly for M3/M4) tells to unwind the stack. see e.g.: arm.com

For this I logically need to change the basic implementation of the HardFault Handler.

HardFault_Handler\
                PROC
                MRS r7,MSP
                EXPORT  HardFault_Handler              [WEAK]
                B       .
                ENDP

after adding the line MRS r7,MSP the HardHandler does not get called any more and the application keeps running.

The same happens when I try to implement my own HardFault_handler in c.

I have no clue. Thanks in advance!

greetings