PVS-Studio warning V547 for Asterisk

static int ebl_callback(....)
{
  unsigned int i;
  ....
  if ((i = dn_expand((unsigned char *)fullanswer,
     (unsigned char *)answer + len,
     (unsigned char *)answer, c->apex, sizeof(c->apex) - 1)) < 0)
  {
    ast_log(LOG_WARNING, "Failed to expand hostname\n");
    return 0;
  }
}

This suspicious code was found in Asterisk project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression is always false. Unsigned type value is never < 0. enum.c 309

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