diff --git a/tests/gzip.test b/tests/gzip.test
index 24bd01ed2..b458307e1 100644
--- a/tests/gzip.test
+++ b/tests/gzip.test
@@ -76,3 +76,6 @@ testing "permissions/times preservation" \
     "gzip -k f1 && TZ=UTC stat -c '%a %Y' f1 && stat -c '%a %X %Y' f1.gz" \
     "411 544413660\n411 252558240 544413660\n" "" ""
 rm -f f1 f1.gz
+
+testing "reject non-gzip" "gzip -dc $FILES/blkid/msdos.bz2 2>/dev/null ||
+    echo rejected" "rejected\n" "" ""
diff --git a/toys/lsb/gzip.c b/toys/lsb/gzip.c
index 3038495e9..049285afe 100644
--- a/toys/lsb/gzip.c
+++ b/toys/lsb/gzip.c
@@ -78,6 +78,7 @@ static int do_deflate(int in_fd, int out_fd, int dd, int level)
   }
   if (!(gz = gzdopen(dd ? in_fd : out_fd, b))) perror_exit("gzdopen");
   if (dd) {
+    if (gzdirect(gz)) error_exit("not gzip");
     while ((len = gzread(gz, toybuf, sizeof(toybuf))) > 0)
       if (len != writeall(out_fd, toybuf, len)) break;
   } else {