PVS-Studio warning V605 for Virtualdub

#define ICERR_CUSTOM -400L

static const char *GetVCMErrorString(uint32 icErr) {
  ....
  if (icErr <= ICERR_CUSTOM)
    err = "A codec-specific error occurred.";
  ....
}

This suspicious code was found in Virtualdub project by PVS-Studio static code analyzer.
Warning message is:
V605 Consider verifying the expression: icErr <= - 400L. An unsigned value is compared to the number -400. system error_win32.cpp 54

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