Missing warning "Type cast is redundant"

Hello

I am using ReSharper 4.0. I have the following code in one of our unit
tests:

01 PrefixSuffixController controller = GetController();
02 controller.Create(null as SubscriptionPrefixSuffixRedirection);

In line 02 no "Type cast is redundant" is shown where null will be casted to
SubscriptionPrefixSuffixRedirection. This only happens because the method
signature of the Create() method is as follows:

public PrefixSuffixMapping Create(PrefixSuffixMapping prefixSuffixMapping)

SubscriptionPrefixSuffixRedirection is a specialisation of the
PrefixSuffixMapping class. Shouldn't this show a "type cast is redundant"
warning?

Gabriel Lozano-Moran