std::string find_first_of and replace

My code below is not behaving as expected. I want to transform
"11/21/13" into "11-21-13"
My loop never exits and the first time through pos = 1, when I would
think it should be 2. Then after the first replace the debugger says my
string is
""1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1/22/2001"
and I don't know where the heck 1x is coming from.

Any hints?

#include <iostream>
#include <string>

//--------------------------------------------------------------------------------------------------
bool Transform_DatetimeString(const std::string & fromXml, std::string &
toSql)
{
    toSql = fromXml;

    std::string::size_type pos = std::string::npos;

    while( pos = toSql.find_first_of('/') != std::string::npos )
    {
        toSql = toSql.replace(pos, 1, '-', 1);
    }

    return true;
}

//--------------------------------------------------------------------------------------------------
int main()
{
    const std::string test = "11/21/13";

    std::string output;
    if( !Transform_DatetimeString(test, output) )
    {
        return 1;
    }

    std::cout << output << std::endl;

    return 0;
}

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