PVS-Studio warning V547 for Firefox

PRBool
nsIEProfileMigrator::TestForIE7()
{
  ...
  PRUint32 index = ieVersion.FindChar('.', 0);
  if (index < 0)
    return PR_FALSE;
  ...
}

This suspicious code was found in Firefox project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression 'index < 0' is always false. Unsigned type value is never < 0. nsieprofilemigrator.cpp 622

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