diff --git a/libtiff/tif_unix.c b/libtiff/tif_unix.c
index 874f1feb..bea1ef78 100644
--- a/libtiff/tif_unix.c
+++ b/libtiff/tif_unix.c
@@ -162,7 +162,7 @@ _tiffMapProc(thandle_t fd, void** pbase, toff_t* psize)
 {
 	uint64 size64 = _tiffSizeProc(fd);
 	tmsize_t sizem = (tmsize_t)size64;
-	if ((uint64)sizem==size64) {
+	if (size64 && (uint64)sizem==size64) {
 		fd_as_handle_union_t fdh;
 		fdh.h = fd;
 		*pbase = (void*)