equations as constraints - error

I try to set up a equation as constraint in a topology optimization. I created a 'dequations': f(f1,f2,f3,f4)=abs((max(f1,f2,f3,f4))-(min(f1,f2,f3,f4)))

It should calculate the difference between the extrems of four displacement responses.

Next I created 4 displacement responses (disp1, disp2, disp3, disp4).

After that I created a function response, edited it: I chose 'RESPONSES' (nresponses =4) and chose the four displacement responses.

In the next step I created a constraint with an upper bound for my function.

When I try to optimize my model I get the following error:' *** ERROR # 1810 *** Inconsistent number of responses on DRESP2 card 5. This error occurs in module 'dppdrv1'.'

Where is my mistake?