error: invalid conversion from ‘char**’ to ‘const char**’ [-fpermissive]

Question

I've this class:

 struct FoundAlphabet {

    FoundAlphabet( char **o, CharStartEnd se) : output(o), start_end(se) {
    }
    const char ** output;
    CharStartEnd start_end;
} ;

I'm getting this error:

cc -g -c  -I -fpermissive -c readimage.cpp -o readimage.o
readimage.cpp: In constructor ‘FoundAlphabet::FoundAlphabet(char**, CharStartEnd)’:
readimage.cpp:32:72: error: invalid conversion from ‘char**’ to ‘const char**’ [-fpermissive]

Yes the first parameter I'm passing is char ** but conversion to const char ** should not be a problem, why is it complaining?

What should I do to solve this error?