[Freesurfer] Question about write_surf.m

Hi freesurfers!

I'm trying to run the function write_surf in matlab, as far as I understand this function is to write a surface to analyze in freesurfer from matlab data. I'm not sure if the extension of the file name has to be different, so I need your help to solve this simple (I guess) problem.

The first line is what I'm trying to run and the next lines show me the error.

*write_surf('prueba.nofix/', vertices, faces);*

*...subtracting 1 from face indices for FreeSurfer compatibility*.

*Error using fwriteInvalid file identifier. Use fopen to generate a valid file identifier.Error in fwrite3 (line 29)fwrite(fid, b1, 'uchar') ;Error in write_surf (line 38)fwrite3(fid, TRIANGLE_FILE_MAGIC_NUMBER);* thank you in advance for your help! Dulce