Properly return a unique ptr

I am writing a String class MyString (yes, as homework) and have to offer a toCString method which returns a unique_ptr<char[]> (and not a Vector). Unfortunately I fail when returning the pointer to the caller: The result is always filled with wrong content - it seems that I create the pointer and/or the character array on the stack.

unique_ptr<char[]> MyString::toCString() const {
     char *characters = new char[m_len];
     char *thisString = m_string.get();
     for (int i = 0; i < m_len; i++) {
         characters[i] = *(thisString + m_start + i);
     }
     const unique_ptr<char[], default_delete<char[]>> &cString = unique_ptr<new char[m_len]>(characters);
     return cString;
}

When debugging I always get expected behaviour. Problems only occur on callers site. Where is my mistake?