PVS-Studio warning V547 for Firebird

static processing_state add_row(TEXT* tabname)
{
  ....
  unsigned varLength, scale;
  ....
  scale = msg->getScale(fbStatus, i);
  ....
  if (scale < 0)
  ....
}

This suspicious code was found in Firebird project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression 'scale < 0' is always false. Unsigned type value is never < 0. isql.cpp 3716

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/