Casting c_str() only works for short strings

I'm using a C library in C++ and wrote a wrapper. At one point I need to convert an std::string to a c-style string. There is a class with a function, which returns a string. Casting the returned string works if the string is short, otherwise not. Here is a simple and reduced example illustrating the issue:

#include <iostream>
#include <string>

class StringBox {
public:
  std::string getString() const { return text_; }

  StringBox(std::string text) : text_(text){};

private:
  std::string text_;
};

int main(int argc, char **argv) {
  const unsigned char *castString = NULL;
  std::string someString = "I am a loooooooooooooooooong string";  // Won't work
  // std::string someString = "hello";  // This one works

  StringBox box(someString);

  castString = (const unsigned char *)box.getString().c_str();
  std::cout << "castString: " << castString << std::endl;

  return 0;
}

Executing the file above prints this to the console:

castString:

whereas if I swap the commenting on someString, it correctly prints

castString: hello

How is this possible?