GMP can't handle leading plus sign?

[Bruce M. Axtens]

Consider the following code

    // BOGP.cpp : Defines the entry point for the console application.
    //

    #include "stdafx.h"
    #include "gmp-static\gmp.h"
    #include <stdlib.h>         /* For _MAX_PATH definition */
    #include <stdio.h>
    #include <malloc.h>


    #define F(x) mpf_t x; mpf_init( x );

    int main(int argc, char* argv[])
    {
        F(foo);
        char * buff;
        mp_exp_t exp;

        mpf_init_set_str( foo, "123", 10 );
        buff = mpf_get_str(NULL, &exp, 10, 0, foo);
        puts( buff );
        puts("\n");
        free(buff);

        mpf_init_set_str( foo, "-123", 10 );
        buff = mpf_get_str(NULL, &exp, 10, 0, foo);
        puts( buff );
        puts("\n");
        free(buff);

        mpf_init_set_str( foo, "+123", 10 );
        buff = mpf_get_str(NULL, &exp, 10, 0, foo);
        puts( buff );
        puts("\n");
        free( buff );

    }

In the first instance, after the mpf_get_str call, buff contains "123".
In the second, buff contains "-123".
But in the third, buff contains an empty string ("").

This is using GMP 4.2.4. Maybe I need to look in the manual again, but I 
would have thought that a leading "+" would have been handled as readily 
as a leading "-".

I'm a tad confused.

Kind regards,
Bruce.