Ambiguous overload for 'operator='

I am trying to compile some C++ code from the Festival project. When I compile Festival, I get the following errors:

Making in directory ./src ...
Making in directory src/arch ...
Making in directory src/arch/festival ...
Making in directory src/modules ...
Making in directory src/modules/rxp ...
Making in directory src/modules/clunits ...
Making in directory src/modules/clustergen ...
Making in directory src/modules/MultiSyn ...
Making in directory src/modules/MultiSyn/inst_tmpl ...
Making in directory src/modules/hts_engine ...
Making in directory src/modules/diphone ...
gcc -c -g -I../include -I../../../src/include -I../../../../speech_tools/include di_io.cc
di_io.cc: In function ‘void load_index(DIPHONE_DATABASE*)’:
di_io.cc:111: error: ambiguous overload for ‘operator=’ in ‘line = EST_TokenStream::get_upto_eoln()()’
../../../../speech_tools/include/EST_String.h:477: note: candidates are: EST_String& EST_String::operator=(const char*) <near match>
../../../../speech_tools/include/EST_String.h:479: note:                 EST_String& EST_String::operator=(char) <near match>
../../../../speech_tools/include/EST_String.h:481: note:                 EST_String& EST_String::operator=(const EST_String&) <near match>
make[3]: *** [di_io.o] Error 1
make[2]: *** [diphone] Error 2
make[1]: *** [modules] Error 2
make: *** [src] Error 2  

The function where the error occurs:

static void load_index(DIPHONE_DATABASE *database)
{
    EST_TokenStream ts;
    int i;
    EST_String line;

    if (ts.open(database->index_file) == -1)
    {
    cerr << "Diphone: Can't open file " << database->index_file << endl;
    festival_error();
    }

    for (i=0; (!ts.eof()) && (i<database->ndiphs);)
    {
    line = ts.get_upto_eoln();   //this is di_io.cc:111
    if ((line.length() > 0) && (line[0] != ';'))
    {
        EST_TokenStream ls;
        ls.open_string(line);
        database->indx[i]->diph = wstrdup(ls.get().string());
        database->indx[i]->file = wstrdup(ls.get().string());
        database->indx[i]->beg = atof(ls.get().string());
        database->indx[i]->mid = atof(ls.get().string());
        database->indx[i]->end = atof(ls.get().string());
        ls.close();
        i++;
    }
    }

    if (i == database->ndiphs)
    {
    cerr << "Diphone: too many diphones in DB" << endl;
    festival_error();
    }

    database->nindex = i;
    database->ndiphs = i;

    ts.close();
}

How can I get rid of the above error?

I'll assume that the non-standard types you're using are from the speech-tools library, documented here, since that was what I found when I googled the class names. If that's wrong, please update the question to indicate where they come from.

I'll also assume that the error line (line 111 of di_io.cc) is the one that says:

line = ts.get_upto_eoln();

since that's the only one I can see that could cause that error message; again, if it's a different line please update the question.

EST_TokenStream::get_upto_eoln returns EST_Token. You are trying to assign it to a variable of a different type (EST_String), and there is no implicit conversion.

You can convert the function result to EST_String using its string function:

line = ts.get_upto_eoln().string();