Template class and 'invalid use of incomplete type' error C++

I have a template Matrix class and I used a std::vector< std::vector<T> > to store data. I need to specialize some methods for complex matrix, for example:

template <typename T>
bool Matrix<T>::is_hermitian() const
{
    if(!(*this).is_squared())
        return false;
    for(int r=0;r<rows_;r++)
        for(int c=0;c<columns_;c++)
            if(mat[r][c]!=mat[c][r])
                return false;
    return true;
}

For the specialized method I thought something like that:

template <typename T>
bool Matrix<std::complex<T> >::is_hermitian() const
{
    if(!(*this).is_squared())
        return false;
    for(int r=0;r<rows_;r++)
        for(int c=0;c<columns_;c++)
            if(mat[r][c]!=std::conj(mat[c][r]))
                return false;
    return true;
}

But the compiler returns me an 'invalid use of incomplete type' error.

I instantiated at the end of the .cpp file a bunch of class that I could be using in the main program:

template class Matrix<int>;
template class Matrix<double>;
template class Matrix<float>;
template class Matrix< std::complex<float> >;
template class Matrix< std::complex<int> >;

How can I implement one method for all std::complex<T> type?

And if you know how to replace the last two instance with a Matrix< std::complex<T> > sort of thing I will be very thankful.