"template argument deduction/substitution failed" for Eigen Matrix

I have the following function:

template <typename DATA_T, std::size_t K, std::size_t F>                                                                                                                                                      
   void write_raw_file(const std::string& filename, Eigen::Matrix<DATA_T, K, F>& data) {                                                                                                                         
      std::fstream out_file = std::fstream(filename, std::ios::out | std::ios::binary);                                                                                                                          

      for(std::size_t i = 0; i < data.rows(); i++) {
         for(std::size_t j = 0; j < data.cols(); j++) { 
            out_file.write((char*) &data(i,j), sizeof(DATA_T));                                                                                                                                                                                                                                                                                                                               
         }
      }

      out_file.close();
   }

I'm calling it with:

std::string filename = "my_file.raw";
Eigen::Matrix<DATA_T, K, static_cast<std::size_t>(1)> my_mat;
write_raw_file(filename, my_mat);

It appears that the compiler can't match the template types as I get this error:

error: no matching function for call to ‘write_raw_file(std::string&, Eigen::Matrix<float, 128, 1, 0, 128, 1>&)’
note: candidate: ‘template<class DATA_T, long unsigned int K, long unsigned int F> void write_raw_file(const string&, Eigen::Matrix<DATA_T, K, F>&)’
void write_raw_file(const std::string& filename, Eigen::Matrix<DATA_T, K, F>& data) {
note:   template argument deduction/substitution failed:
note:   mismatched types ‘long unsigned int’ and ‘int’
write_raw_file(filename, my_mat);
~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~

I'm fairly new to C++/templates. So this is kinda confusing. I've searched for similar situations but this is a pretty basic use-case and I'm at a loss as to why it's failing.

I've called the function with a std::string& and a Eigen::Matrix<DATA_T, K, F>& type.