Error C2555 with pointer to class with no template instantiation?

Error C2555

c:\C++ projects\stat1\stdmatrix_adapt.h(41) : error C2555:
'std_tools::Matrix_adapter<T>::at': overriding virtual function return
type differs and is not covariant from 'ple::imtx_impl<T>::at'

//in my program: the derived class
template <class T>
class Matrix_adapter : public ple::imtx_impl<T> {
protected:
    Matrix* m_;
public:
    virtual const T& at(mtx_size i0, mtx_size i1) const //mtx_size -->
unsigned int
        {return m_->at(i0, i1);}
}
// the base class
template <class T>
class imtx_impl : public imtx<T> {
    virtual const T& at(mtx_size i0, mtx_size i1) const=0;

/*Notice Matrix_adapter.at returns a pointer to another class Matrix
m_
The problem arises as this class Matrix has no template instantiation
and i set the virtual function at return as double, the same as T */

class Matrix : public GeneralMatrix
public:{
const double& at(unsigned int pos0, unsigned int pos1) const { return
v_.at(c_(pos0, pos1)); }
}

// However in my main program initiating T as double in the derived
class still gives the error C2555.

Matrix_adapter <double> matx;

Am at loss what to do now.. any people care to help... ?