Why does ptr_fun(tolower) doesn't compile in mingw?

I'm trying to compile the following program using Qt Creator 2.0.1:

void f()
{
    string a = "abc";
    transform(a.begin(), a.end(), a.begin(), ptr_fun(tolower));
}

mingw throws the following error:

no matching function for call to ptr_fun( < unresolved overloaded function type > )

That function compiles fine with VC++ 2010 Express. What is wrong with mingw?

Thanks.