It turns out that it's not all that uncommon to have a C++ override of, say, memcpy that receives a structure (or two) by reference (or by value, if it's being copied from) and copies memory from it (or into it, if it's passed by reference). In this case the argument will be of structure type (recall that expressions of reference type do not exist: instead, C++ classifies expressions into prvalues and lvalues and xvalues).
In this scenario we crash because we are trying to assume that, say, a memory region is equal to an empty CompoundValue (the non-lazy one; this is what makeZeroVal() return for compound types and it represents prvalue of an object that is initialized with an empty initializer list).
Add defensive checks. We should probably extend CallDescription so that it encapsulated these checks and we were always sure that this is the function we're looking for.