--- msdosfs_fat.c.orig  Wed Jun 16 11:16:37 2004
+++ msdosfs_fat.c       Wed Jun 16 11:15:41 2004
@@ -434,8 +434,9 @@
                        for (cn = 0; cn < pmp->pm_maxcluster; cn += N_INUSEBITS)
                                if (pmp->pm_inusemap[cn / N_INUSEBITS] != (u_int)-1)
                                        break;
-                       pmp->pm_nxtfree = cn
-                + ffs(pmp->pm_inusemap[cn / N_INUSEBITS]^(u_int)-1) - 1;
+                       pmp->pm_nxtfree = (cn < pmp->pm_maxcluster)?
+                       (cn + ffs(pmp->pm_inusemap[cn / N_INUSEBITS]^(u_int)-1) - 1)
+                       :0;
                }
                if (bread(pmp->pm_devvp, pmp->pm_fsinfo, fsi_size(pmp),
                    NOCRED, &bpn) != 0) {