diff --git a/sys/fs/msdosfs/msdosfs_fat.c b/sys/fs/msdosfs/msdosfs_fat.c
index 3c17f00b971e..cf03e00654e2 100644
--- a/sys/fs/msdosfs/msdosfs_fat.c
+++ b/sys/fs/msdosfs/msdosfs_fat.c
@@ -380,6 +380,8 @@ usemap_alloc(struct msdosfsmount *pmp, u_long cn)
 
 	MSDOSFS_ASSERT_MP_LOCKED(pmp);
 
+	KASSERT((pmp->pm_flags & MSDOSFSMNT_RONLY) == 0,
+	    ("usemap_alloc on ro msdosfs mount"));
 	KASSERT((pmp->pm_inusemap[cn / N_INUSEBITS] & (1 << (cn % N_INUSEBITS)))
 	    == 0, ("Allocating used sector %ld %ld %x", cn, cn % N_INUSEBITS,
 		(unsigned)pmp->pm_inusemap[cn / N_INUSEBITS]));
@@ -394,6 +396,8 @@ usemap_free(struct msdosfsmount *pmp, u_long cn)
 {
 
 	MSDOSFS_ASSERT_MP_LOCKED(pmp);
+	KASSERT((pmp->pm_flags & MSDOSFSMNT_RONLY) == 0,
+	    ("usemap_free on ro msdosfs mount"));
 	pmp->pm_freeclustercount++;
 	pmp->pm_flags |= MSDOSFS_FSIMOD;
 	KASSERT((pmp->pm_inusemap[cn / N_INUSEBITS] & (1 << (cn % N_INUSEBITS)))
@@ -675,6 +679,8 @@ chainalloc(struct msdosfsmount *pmp, u_long start, u_long count,
 	u_long cl, n;
 
 	MSDOSFS_ASSERT_MP_LOCKED(pmp);
+	KASSERT((pmp->pm_flags & MSDOSFSMNT_RONLY) == 0,
+	    ("chainalloc on ro msdosfs mount"));
 
 	for (cl = start, n = count; n-- > 0;)
 		usemap_alloc(pmp, cl++);