diff --git a/sys/kern/vfs_mount.c b/sys/kern/vfs_mount.c
--- a/sys/kern/vfs_mount.c
+++ b/sys/kern/vfs_mount.c
@@ -1313,7 +1313,7 @@
 	void *bufp;
 	struct mount *mp;
 	int error, export_error, i, len, fsid_up_len;
-	uint64_t flag;
+	uint64_t flag, mnt_union;
 	gid_t *grps;
 	fsid_t *fsid_up;
 	bool vfs_suser_failed;
@@ -1395,6 +1395,7 @@
 		vfs_deleteopt(*optlist, "fsid");
 	}
 
+	mnt_union = 0;
 	MNT_ILOCK(mp);
 	if ((mp->mnt_kern_flag & MNTK_UNMOUNT) != 0) {
 		MNT_IUNLOCK(mp);
@@ -1416,6 +1417,11 @@
 		mp->mnt_flag |= MNT_UPDATE;
 	} else {
 		mp->mnt_flag &= ~MNT_UPDATEMASK;
+		if ((mp->mnt_flag & MNT_UNION) == 0 &&
+		    (fsflags & MNT_UNION) != 0) {
+			fsflags &= ~MNT_UNION;
+			mnt_union = MNT_UNION;
+		}
 		mp->mnt_flag |= fsflags & (MNT_RELOAD | MNT_FORCE | MNT_UPDATE |
 		    MNT_SNAPSHOT | MNT_ROOTFS | MNT_UPDATEMASK | MNT_RDONLY);
 		if ((mp->mnt_flag & MNT_ASYNC) == 0)
@@ -1519,6 +1525,7 @@
 	if (error == 0) {
 		mp->mnt_flag &= ~(MNT_UPDATE | MNT_RELOAD | MNT_FORCE |
 		    MNT_SNAPSHOT);
+		mp->mnt_flag |= mnt_union;
 	} else {
 		/*
 		 * If we fail, restore old mount flags. MNT_QUOTA is special,