PVS-Studio warning V530 for Micro-Manager

int HalogenLamp::SetIntensity(long intensity)
{
  ....
  command_stream.str().c_str();
  ....
}

This suspicious code was found in Micro-Manager project by PVS-Studio static code analyzer.
Warning message is:
V530 The return value of function 'c_str' is required to be utilized. ZeissCAN.cpp 1553

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/