diff --git a/lfs.c b/lfs.c
index 951df6ef..13c37d79 100644
--- a/lfs.c
+++ b/lfs.c
@@ -5521,8 +5521,6 @@ typedef struct lfsr_mtree_traversal {
     uint8_t tortoise_power;
     lfs_size_t tortoise_step;
     lfsr_mpair_t tortoise_mpair;
-    lfs_block_t tortoise_mtree_block;
-    lfs_size_t tortoise_mtree_trunk;
 } lfsr_mtree_traversal_t;
 
 enum {
@@ -5541,25 +5539,23 @@ static int lfsr_mtree_traversal_next(lfs_t *lfs,
         lfsr_mtree_traversal_t *traversal,
         lfs_size_t *mid_, lfsr_tag_t *tag_, lfsr_data_t *data_) {
     // detect cycles with Brent's algorithm
-    if (traversal->mdir.rbyd.trunk != 0
-            && lfsr_mpair_eq(lfsr_mdir_mpair(&traversal->mdir),
-                traversal->tortoise_mpair)
-            && traversal->mtraversal.branch.block
-                == traversal->tortoise_mtree_block
-            && traversal->mtraversal.branch.trunk
-                == traversal->tortoise_mtree_trunk) {
-        LFS_ERROR("Cycle detected during mtree traversal");
-        return LFS_ERR_CORRUPT;
+    if (traversal->mtraversal.branch.trunk == 0) {
+        if (traversal->mdir.rbyd.trunk != 0
+                && lfsr_mpair_eq(lfsr_mdir_mpair(&traversal->mdir),
+                    traversal->tortoise_mpair)) {
+            LFS_ERROR("Cycle detected during mtree traversal "
+                    "(0x{%"PRIx32",%"PRIx32"})",
+                    traversal->mdir.rbyd.block, traversal->mdir.other_block);
+            return LFS_ERR_CORRUPT;
+        }
+        if (traversal->tortoise_step
+                == ((lfs_size_t)1 << traversal->tortoise_power)) {
+            traversal->tortoise_mpair = lfsr_mdir_mpair(&traversal->mdir);
+            traversal->tortoise_step = 0;
+            traversal->tortoise_power += 1;
+        }
+        traversal->tortoise_step += 1;
     }
-    if (traversal->tortoise_step
-            == ((lfs_size_t)1 << traversal->tortoise_power)) {
-        traversal->tortoise_mpair = lfsr_mdir_mpair(&traversal->mdir);
-        traversal->tortoise_mtree_block = traversal->mtraversal.branch.block;
-        traversal->tortoise_mtree_trunk = traversal->mtraversal.branch.trunk;
-        traversal->tortoise_step = 0;
-        traversal->tortoise_power += 1;
-    }
-    traversal->tortoise_step += 1;
 
     // new traversal? start with 0x{0,1}
     //
@@ -5642,7 +5638,7 @@ static int lfsr_mtree_traversal_next(lfs_t *lfs,
                     && rid == -1
                     && lfsr_tag_suptype(tag) == LFSR_TAG_STRUCT) {
                 if (tag != LFSR_TAG_MDIR && tag != LFSR_TAG_BTREE) {
-                    LFS_ERROR("Weird mstruct? 0x%"PRIx32, tag);
+                    LFS_ERROR("Weird mstruct? (0x%"PRIx32")", tag);
                     return LFS_ERR_CORRUPT;
                 }
 
@@ -5685,7 +5681,7 @@ static int lfsr_mtree_traversal_next(lfs_t *lfs,
             if (err) {
                 if (err == LFS_ERR_CORRUPT) {
                     LFS_ERROR("Corrupted rbyd during mtree traversal "
-                            "(rbyd=0x%"PRIx32".%"PRIx32", 0x%08"PRIx32")",
+                            "(0x%"PRIx32".%"PRIx32", 0x%08"PRIx32")",
                             branch->block, branch->trunk, branch->crc);
                 }
                 return err;
@@ -5698,7 +5694,7 @@ static int lfsr_mtree_traversal_next(lfs_t *lfs,
             // crc check and trunk check
             if (branch_.crc != branch->crc) {
                 LFS_ERROR("Checksum mismatch during mtree traversal "
-                        "(rbyd=0x%"PRIx32".%"PRIx32", "
+                        "(0x%"PRIx32".%"PRIx32", "
                         "0x%08"PRIx32" != 0x%08"PRIx32")",
                         branch->block, branch->trunk,
                         branch_.crc, branch->crc);
@@ -5754,7 +5750,7 @@ static int lfsr_mtree_traversal_next(lfs_t *lfs,
         return 0;
 
     } else {
-        LFS_ERROR("Weird mtree entry? 0x%"PRIx32, tag);
+        LFS_ERROR("Weird mtree entry? (0x%"PRIx32")", tag);
         return LFS_ERR_CORRUPT;
     }
 }
diff --git a/tests/test_mtree.toml b/tests/test_mtree.toml
index 657920b7..856c07c2 100644
--- a/tests/test_mtree.toml
+++ b/tests/test_mtree.toml
@@ -3742,3 +3742,63 @@ code = '''
     }
 '''
 
+
+## Cycle detection? ##
+
+# test that our cycle detector at least works in common cases
+[cases.test_mtree_traversal_mroot_cycle]
+in = 'lfs.c'
+code = '''
+    lfs_t lfs;
+    lfsr_format(&lfs, cfg) => 0;
+    lfsr_mount(&lfs, cfg) => 0;
+
+    uint8_t buffer[LFSR_MPAIR_DSIZE];
+    lfs_ssize_t d = lfsr_mpair_todisk(&lfs, LFSR_MPAIR(0, 1), buffer);
+    assert(d >= 0);
+    lfsr_mdir_commit(&lfs, &lfs.mroot, &(lfs_ssize_t){-1}, LFSR_ATTRS(
+            LFSR_ATTR(-1, MROOT, 0, buffer, d))) => 0;
+
+    // technically, cycle detection only needs to work when we're validating
+    lfsr_mtree_traversal_t traversal = LFSR_MTREE_TRAVERSAL_INIT(
+            LFSR_MTREE_TRAVERSAL_VALIDATE);
+
+    for (lfs_block_t i = 0;; i++) {
+        // assert that we detect the cycle in a reasonable number of iterations
+        assert(i < 1024);
+
+        lfs_size_t mid_;
+        lfsr_tag_t tag_;
+        lfsr_data_t data_;
+        int err = lfsr_mtree_traversal_next(&lfs, &traversal,
+                &mid_, &tag_, &data_);
+        assert(!err || err == LFS_ERR_CORRUPT);
+        if (err == LFS_ERR_CORRUPT) {
+            break;
+        }
+
+        if (tag_ == LFSR_TAG_BTREE) {
+            lfsr_rbyd_t *branch = (lfsr_rbyd_t *)data_.buf.buffer;
+            printf("traversal: %d 0x%x btree 0x%x.%x\n",
+                    mid_,
+                    tag_,
+                    branch->block, branch->trunk);
+        } else if (tag_ == LFSR_TAG_MDIR) {
+            lfsr_mdir_t *mdir = (lfsr_mdir_t*)data_.buf.buffer;
+            printf("traversal: %d 0x%x mdir 0x{%x,%x}\n",
+                    mid_,
+                    tag_,
+                    mdir->rbyd.block, mdir->other_block);
+        } else {
+            // this shouldn't happen
+            printf("traversal: %d 0x%x %d\n",
+                    mid_,
+                    tag_,
+                    lfsr_data_size(data_));
+            assert(false);
+        }
+    }
+
+    lfsr_unmount(&lfs) => 0;
+'''
+