Is Simulink's for iterator subsystem compatible with derivative blocs?

Hi,

I need to use one of my subsytems several times at each time step so I put it in a for iterator subsystem but I get the following error:

'.../Subsystem/Derivative1' has sample time [0, 0]. Only constant (inf) or inherited (-1) sample times are allowed in iterator subsystem '.../Subsystem'.

Apparently, the trouble comes from one of the derivative blocks in the subsystem. But Derivative1 does have sample time set to inf so I don't understand what makes Simulink crash.