Bug in ROS_INFO_STREAM?

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I have a stringstream that I'm using to store some debug information. Every so often, I try to print it out. I have found that

string s = ss.str();
ROS_INFO_STREAM(s);

works just as expected, the contents of ss are displayed in a message. But an operation that I feel should be identical,

ROS_INFO_STREAM( (ss.str()) );

results in nothing at all being displayed, except of course the usual ROS_INFO header "[ INFO]..."

Any ideas on why this could be?