diff --git a/sbin/fsck_msdosfs/boot.c b/sbin/fsck_msdosfs/boot.c
index b1760c5..9e043b0 100644
--- a/sbin/fsck_msdosfs/boot.c
+++ b/sbin/fsck_msdosfs/boot.c
@@ -176,10 +176,14 @@ readboot(int dosfs, struct bootblock *boot)
 		/* Check backup bpbFSInfo?					XXX */
 	}
 
-	boot->ClusterOffset = (boot->bpbRootDirEnts * 32 +
-	    boot->bpbBytesPerSec - 1) / boot->bpbBytesPerSec +
-	    boot->bpbResSectors + boot->bpbFATs * boot->FATsecs -
-	    CLUST_FIRST * boot->bpbSecPerClust;
+	if (boot->bpbBytesPerSec > 0)
+		boot->ClusterOffset = (boot->bpbRootDirEnts * 32 +
+		    boot->bpbBytesPerSec - 1) / boot->bpbBytesPerSec;
+	else
+		boot->ClusterOffset = 0;
+	boot->ClusterOffset += boot->bpbResSectors;
+	boot->ClusterOffset += boot->bpbFATs * boot->FATsecs;
+	boot->ClusterOffset -= CLUST_FIRST * boot->bpbSecPerClust;
 
 	if (boot->bpbBytesPerSec % DOSBOOTBLOCKSIZE_REAL != 0) {
 		pfatal("Invalid sector size: %u", boot->bpbBytesPerSec);