error C2679: binary '=' : no operator found which takes a right-hand operator

I am using STL internals in my application. But when I am compiling my Application in debug mode it is going fine but in Relase mode I am getting compilation error.

Can anyone please provide me solution for this one.
error is as follows:

error C2679: binary '=' : no operator found which takes a right-hand operand of type 'stdext::_Hash<_Traits>::_List_position' (or there is no acceptable conversion)

size_type _Bucket = SOME VALUE ASSINED.
iterator _Where;
_Where = _Vec[_Bucket]; - Error is at this line.

Thanks,