Python ctypes: how to free memory? Getting invalid pointer error

I want to get some string from a C/C++ library with ctypes into python. My code looks like this:

Code in lib:

const char* get(struct something *x) 
{
    [...]
    // buf is a stringstream
   return strdup(buf.str().c_str());
}

void freeme(char *ptr)
{
    free(ptr);
}

Python code:

fillprototype(lib.get, c_char_p, POINTER(some_model)])
fillprototype(lib.freeme, None, [c_char_p])

// what i want to do here: get a string into python so that i can work
// with it and release the memory in the lib.
c_str = lib.get(some_model)
y = ''.join(c_str)
lib.freeme(c_str) 

If i print() c_str, everything is there. Problem is (or seems to be) in the last Python line. I cannot free the memory - the library is getting a wrong pointer. What I am doing wrong here? (And please don't suggest boost::python or so).

*** glibc detected *** python2: munmap_chunk(): invalid pointer: 0x00000000026443fc ***