diff --git a/third_party/libtiff/tif_pixarlog.c b/third_party/libtiff/tif_pixarlog.c
index 626409058..e72cc0be9 100644
--- a/third_party/libtiff/tif_pixarlog.c
+++ b/third_party/libtiff/tif_pixarlog.c
@@ -799,7 +799,7 @@ PixarLogDecode(TIFF* tif, uint8* op, tmsize_t occ, uint16 s)
 		return (0);
 	}
 	/* Check that we will not fill more than what was allocated */
-	if ((tmsize_t)sp->stream.avail_out > sp->tbuf_size)
+        if (sp->tbuf_size < 0 || sp->stream.avail_out > (uInt) sp->tbuf_size)
 	{
 		TIFFErrorExt(tif->tif_clientdata, module, "sp->stream.avail_out > sp->tbuf_size");
 		return (0);