diff --git a/sbin/fsck_msdosfs/boot.c b/sbin/fsck_msdosfs/boot.c
index 5816b986f69..61920e68af5 100644
--- a/sbin/fsck_msdosfs/boot.c
+++ b/sbin/fsck_msdosfs/boot.c
@@ -259,12 +259,18 @@ readboot(int dosfs, struct bootblock *boot)
 		return FSFATAL;
 	}
 
-	boot->ClusterOffset = (boot->bpbRootDirEnts * 32 +
+	boot->FirstCluster = (boot->bpbRootDirEnts * 32 +
 	    boot->bpbBytesPerSec - 1) / boot->bpbBytesPerSec +
-	    boot->bpbResSectors + boot->bpbFATs * boot->FATsecs -
-	    CLUST_FIRST * boot->bpbSecPerClust;
-	boot->NumClusters = (boot->NumSectors - boot->ClusterOffset) /
-	    boot->bpbSecPerClust;
+	    boot->bpbResSectors + boot->bpbFATs * boot->FATsecs;
+
+	if (boot->FirstCluster + boot->bpbSecPerClust > boot->NumSectors) {
+		pfatal("Cluster offset too large (%u clusters)\n",
+		    boot->FirstCluster);
+		return FSFATAL;
+	}
+
+	boot->NumClusters = (boot->NumSectors - boot->FirstCluster) / boot->bpbSecPerClust +
+	    CLUST_FIRST;
 
 	if (boot->flags & FAT32)
 		boot->ClustMask = CLUST32_MASK;
diff --git a/sbin/fsck_msdosfs/dir.c b/sbin/fsck_msdosfs/dir.c
index 2163db049fa..7e459c75d4f 100644
--- a/sbin/fsck_msdosfs/dir.c
+++ b/sbin/fsck_msdosfs/dir.c
@@ -317,7 +317,8 @@ delete(int f, struct bootblock *boot, struct fatEntry *fat, cl_t startcl,
 				break;
 			e = delbuf + endoff;
 		}
-		off = startcl * boot->bpbSecPerClust + boot->ClusterOffset;
+		off = (startcl - CLUST_FIRST) * boot->bpbSecPerClust + boot->FirstCluster;
+
 		off *= boot->bpbBytesPerSec;
 		if (lseek(f, off, SEEK_SET) != off) {
 			perr("Unable to lseek to %" PRId64, off);
@@ -457,7 +458,7 @@ check_subdirectory(int f, struct bootblock *boot, struct dosDirEntry *dir)
 		off = boot->bpbResSectors + boot->bpbFATs *
 			boot->FATsecs;
 	} else {
-		off = cl * boot->bpbSecPerClust + boot->ClusterOffset;
+		off = (cl - CLUST_FIRST) * boot->bpbSecPerClust + boot->FirstCluster;
 	}
 
 	/*
@@ -538,7 +539,7 @@ readDosDirSection(int f, struct bootblock *boot, struct fatEntry *fat,
 			    boot->FATsecs;
 		} else {
 			last = boot->bpbSecPerClust * boot->bpbBytesPerSec;
-			off = cl * boot->bpbSecPerClust + boot->ClusterOffset;
+			off = (cl - CLUST_FIRST) * boot->bpbSecPerClust + boot->FirstCluster;
 		}
 
 		off *= boot->bpbBytesPerSec;
@@ -1069,8 +1070,9 @@ reconnect(int dosfs, struct bootblock *boot, struct fatEntry *fat, cl_t head)
 			lfcl = (lostDir->head < boot->NumClusters) ? lostDir->head : 0;
 			return FSERROR;
 		}
-		lfoff = lfcl * boot->ClusterSize
-		    + boot->ClusterOffset * boot->bpbBytesPerSec;
+		lfoff = (lfcl - CLUST_FIRST) * boot->ClusterSize
+		    + boot->FirstCluster * boot->bpbBytesPerSec;
+
 		if (lseek(dosfs, lfoff, SEEK_SET) != lfoff
 		    || (size_t)read(dosfs, lfbuf, boot->ClusterSize) != boot->ClusterSize) {
 			perr("could not read LOST.DIR");
diff --git a/sbin/fsck_msdosfs/dosfs.h b/sbin/fsck_msdosfs/dosfs.h
index 3d84ea01519..9f1480f17b0 100644
--- a/sbin/fsck_msdosfs/dosfs.h
+++ b/sbin/fsck_msdosfs/dosfs.h
@@ -74,7 +74,7 @@ struct bootblock {
 	u_int32_t NumSectors;		/* how many sectors are there */
 	u_int32_t FATsecs;		/* how many sectors are in FAT */
 	u_int32_t NumFatEntries;	/* how many entries really are there */
-	u_int	ClusterOffset;		/* at what sector would sector 0 start */
+	u_int	FirstCluster;		/* at what sector is Cluster CLUST_FIRST */
 	u_int	ClusterSize;		/* Cluster size in bytes */
 
 	/* Now some statistics: */