diff --git a/lib/libc/sys/mount.2 b/lib/libc/sys/mount.2
index 1b551ed5b07a..58094f2d9e83 100644
--- a/lib/libc/sys/mount.2
+++ b/lib/libc/sys/mount.2
@@ -32,7 +32,7 @@
 .\"     @(#)mount.2	8.3 (Berkeley) 5/24/95
 .\" $FreeBSD$
 .\"
-.Dd July 1, 2003
+.Dd September 8, 2003
 .Dt MOUNT 2
 .Os
 .Sh NAME
@@ -294,15 +294,20 @@ system call may fail with one of the following errors:
 The caller is neither the super-user nor the user who issued the corresponding
 .Fn mount
 call.
-.It Bq Er ENOTDIR
-A component of the path is not a directory.
 .It Bq Er ENAMETOOLONG
-A component of a pathname exceeded 255 characters,
-or an entire path name exceeded 1023 characters.
-.It Bq Er ELOOP
-Too many symbolic links were encountered in translating the pathname.
+The length of the path name exceeded 1023 characters.
 .It Bq Er EINVAL
 The requested directory is not in the mount table.
+.It Bq Er ENOENT
+The file system ID specified using
+.Dv MNT_BYFSID
+was not found in the mount table.
+.It Bq Er EINVAL
+The file system ID specified using
+.Dv MNT_BYFSID
+could not be decoded.
+.It Bq Er EINVAL
+The specified file system is the root file system.
 .It Bq Er EBUSY
 A process is holding a reference to a file located
 on the file system.
diff --git a/sys/kern/vfs_mount.c b/sys/kern/vfs_mount.c
index 7d166f02f45d..efc78745bbbf 100644
--- a/sys/kern/vfs_mount.c
+++ b/sys/kern/vfs_mount.c
@@ -1255,8 +1255,15 @@ unmount(td, uap)
 		mtx_unlock(&mountlist_mtx);
 	}
 	free(pathbuf, M_TEMP);
-	if (mp == NULL)
-		return (ENOENT);
+	if (mp == NULL) {
+		/*
+		 * Previously we returned ENOENT for a nonexistent path and
+		 * EINVAL for a non-mountpoint.  We cannot tell these apart
+		 * now, so in the !MNT_BYFSID case return the more likely
+		 * EINVAL for compatibility.
+		 */
+		return ((uap->flags & MNT_BYFSID) ? ENOENT : EINVAL);
+	}
 
 	/*
 	 * Only root, or the user that did the original mount is