PVS-Studio warning V562 for spvolren

void ppmWrite(char *filename, PPMFile *ppmFile)
{
  ...
  FILE *fp;
  if (! (fp = fopen(filename, "wb")) == -1) {
    perror("opening image file failed");
    exit(1);
  }
  ...
}

This suspicious code was found in spvolren project by PVS-Studio static code analyzer.
Warning message is:
V562 It's odd to compare a bool type value with a value of -1: !(fp = fopen (filename, "wb")) == - 1. spvolren ppm.c 118

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