diff --git a/sys/geom/label/g_label.c b/sys/geom/label/g_label.c
--- a/sys/geom/label/g_label.c
+++ b/sys/geom/label/g_label.c
@@ -394,6 +394,10 @@
 	if (pp->acw > 0)
 		return (NULL);
 
+	/* Skip broken disks that don't set their sector size */
+	if (pp->sectorsize == 0)
+		return (NULL);
+
 	if (strcmp(pp->geom->class->name, mp->name) == 0)
 		return (NULL);
 
diff --git a/sys/geom/label/g_label_ext2fs.c b/sys/geom/label/g_label_ext2fs.c
--- a/sys/geom/label/g_label_ext2fs.c
+++ b/sys/geom/label/g_label_ext2fs.c
@@ -62,10 +62,11 @@
 	pp = cp->provider;
 	label[0] = '\0';
 
-	if ((EXT2FS_SB_OFFSET % pp->sectorsize) != 0)
-		return;
+	KASSERT(pp->sectorsize != 0, ("Tasting a disk with 0 sectorsize"));
 	if (pp->sectorsize < sizeof(*fs))
 		return;
+	if ((EXT2FS_SB_OFFSET % pp->sectorsize) != 0)
+		return;
 
 	fs = g_read_data(cp, EXT2FS_SB_OFFSET, pp->sectorsize, NULL);
 	if (fs == NULL)
diff --git a/sys/geom/label/g_label_iso9660.c b/sys/geom/label/g_label_iso9660.c
--- a/sys/geom/label/g_label_iso9660.c
+++ b/sys/geom/label/g_label_iso9660.c
@@ -52,10 +52,11 @@
 	pp = cp->provider;
 	label[0] = '\0';
 
-	if ((ISO9660_OFFSET % pp->sectorsize) != 0)
-		return;
+	KASSERT(pp->sectorsize != 0, ("Tasting a disk with 0 sectorsize"));
 	if (pp->sectorsize < 0x28 + VOLUME_LEN)
 		return;
+	if ((ISO9660_OFFSET % pp->sectorsize) != 0)
+		return;
 	sector = g_read_data(cp, ISO9660_OFFSET, pp->sectorsize, NULL);
 	if (sector == NULL)
 		return;
diff --git a/sys/geom/label/g_label_reiserfs.c b/sys/geom/label/g_label_reiserfs.c
--- a/sys/geom/label/g_label_reiserfs.c
+++ b/sys/geom/label/g_label_reiserfs.c
@@ -59,10 +59,11 @@
 
 	secsize = cp->provider->sectorsize;
 
-	if ((offset % secsize) != 0)
-		return (NULL);
+	KASSERT(secsize != 0, ("Tasting a disk with 0 sectorsize"));
 	if (secsize < sizeof(*fs))
 		return (NULL);
+	if ((offset % secsize) != 0)
+		return (NULL);
 
 	fs = g_read_data(cp, offset, secsize, NULL);
 	if (fs == NULL)
diff --git a/sys/geom/label/g_label_ufs.c b/sys/geom/label/g_label_ufs.c
--- a/sys/geom/label/g_label_ufs.c
+++ b/sys/geom/label/g_label_ufs.c
@@ -139,6 +139,7 @@
 	label[0] = '\0';
 
 	fs = NULL;
+	KASSERT(pp->sectorsize != 0, ("Tasting a disk with 0 sectorsize"));
 	if (SBLOCKSIZE % pp->sectorsize != 0 || ffs_sbget(cp, &fs, UFS_STDSB,
 	    UFS_NOHASHFAIL | UFS_NOCSUM | UFS_NOMSG, M_GEOM, g_use_g_read_data)
 	    != 0) {