diff --git a/src/atmst/mst/node.py b/src/atmst/mst/node.py
index 1691f02..fba9dae 100644
--- a/src/atmst/mst/node.py
+++ b/src/atmst/mst/node.py
@@ -114,7 +114,7 @@ def deserialise(cls, data: bytes) -> "Self":
 			keys=tuple(keys),
 			vals=tuple(vals)
 		)
-	
+
 	def is_empty(self) -> bool:
 		return self.subtrees == (None,)
 
@@ -128,18 +128,25 @@ def _to_optional(self) -> Optional[CID]:
 
 
 	@cached_property
-	def height(self) -> int:
+	def maybe_height(self) -> Optional[int]:
 		# if there are keys at this level, query one directly
 		if self.keys:
 			return self.key_height(self.keys[0])
-		
+
 		# we're an empty tree
 		if self.subtrees[0] is None:
 			return 0
-		
-		# this should only happen for non-root nodes with no keys
-		raise Exception("cannot determine node height")
-	
+
+		# this should only happen for non-root nodes with no keys (aka an empty intermediate node)
+		return None
+		# NOTE: a Node class cannot see what's below it. You'll need to track
+		# state externally (like NodeWrangler does) if you want to find out
+
+	def definitely_height(self) -> int:
+		if self.maybe_height is None:
+			raise ValueError("indeterminate node height")
+		return self.maybe_height
+
 	def gte_index(self, key: str) -> int:
 		"""
 		find the index of the first key greater than or equal to the specified key
diff --git a/src/atmst/mst/node_store.py b/src/atmst/mst/node_store.py
index 2ec5d73..ebdbead 100644
--- a/src/atmst/mst/node_store.py
+++ b/src/atmst/mst/node_store.py
@@ -52,7 +52,7 @@ def pretty(self, node_cid: Optional[CID]) -> str:
 		if node_cid is None:
 			return "<empty>"
 		node = self.get_node(node_cid)
-		res = f"MSTNode<cid={node.cid.encode('base32')}>(\n{indent(self.pretty(node.subtrees[0]))},\n"
+		res = f"MSTNode<cid={node.cid.encode('base32')}, maybe_height={node.maybe_height}>(\n{indent(self.pretty(node.subtrees[0]))},\n"
 		for k, v, t in zip(node.keys, node.vals, node.subtrees[1:]):
 			res += f"  {k!r} ({MSTNode.key_height(k)}) -> {v.encode('base32')},\n"
 			res += indent(self.pretty(t)) + ",\n"
diff --git a/src/atmst/mst/node_walker.py b/src/atmst/mst/node_walker.py
index 4c9dcb8..ec05f9d 100644
--- a/src/atmst/mst/node_walker.py
+++ b/src/atmst/mst/node_walker.py
@@ -37,19 +37,38 @@ class StackFrame:
 
 	ns: NodeStore
 	stack: List[StackFrame]
+	root_height: int
 	
-	def __init__(self, ns: NodeStore, root_cid: Optional[CID], lpath: Optional[str]=PATH_MIN, rpath: Optional[str]=PATH_MAX, trusted: Optional[bool]=False) -> None:
+	def __init__(self,
+		ns: NodeStore,
+		root_cid: Optional[CID],
+		lpath: str=PATH_MIN,
+		rpath: str=PATH_MAX,
+		trusted: bool=False,
+		root_height: Optional[int]=None
+	) -> None:
 		self.ns = ns
 		self.trusted = trusted
+		node = MSTNode.empty_root() if root_cid is None else self.ns.get_node(root_cid)
+		self.root_height = node.maybe_height if root_height is None else root_height
+		if self.root_height is None:
+			raise ValueError("indeterminate node height - pass it in if you know it")
 		self.stack = [self.StackFrame(
-			node=MSTNode.empty_root() if root_cid is None else self.ns.get_node(root_cid),
+			node=node,
 			lpath=lpath,
 			rpath=rpath,
 			idx=0
 		)]
 	
 	def subtree_walker(self) -> "Self":
-		return NodeWalker(self.ns, self.subtree, self.lpath, self.rpath, self.trusted)
+		return NodeWalker(
+			self.ns,
+			self.subtree,
+			self.lpath,
+			self.rpath,
+			self.trusted,
+			root_height=self.height - 1
+		)
 	
 	@property
 	def frame(self) -> StackFrame:
@@ -57,7 +76,7 @@ def frame(self) -> StackFrame:
 
 	@property
 	def height(self) -> int:
-		return self.frame.node.height
+		return self.root_height - (len(self.stack) - 1)
 
 	@property
 	def lpath(self) -> str:
@@ -91,7 +110,7 @@ def can_go_right(self) -> bool:
 	def right_or_up(self) -> None:
 		if not self.can_go_right:
 			# we reached the end of this node, go up a level
-			self.stack.pop()
+			self.stack.pop() # TODO: check before pop - make empty-stack an unreachable state
 			if not self.stack:
 				raise StopIteration # you probably want to check .final instead of hitting this
 			return self.right_or_up() # we need to recurse, to skip over empty intermediates on the way back up
@@ -110,8 +129,9 @@ def down(self) -> None:
 		subtree_node = self.ns.get_node(subtree)
 
 		if not self.trusted: # if we "trust" the source we can elide this check
-			if subtree_node.height != self.height - 1:
-				raise ValueError("inconsistent subtree height")
+			# the "None" case occurs for empty intermediate nodes
+			if subtree_node.maybe_height is not None and subtree_node.maybe_height != self.height - 1:
+				raise ValueError(f"inconsistent subtree height ({subtree_node.maybe_height}, expected {self.height - 1})")
 
 		self.stack.append(self.StackFrame(
 			node=subtree_node,
diff --git a/src/atmst/mst/node_wrangler.py b/src/atmst/mst/node_wrangler.py
index fcd354a..906abf1 100644
--- a/src/atmst/mst/node_wrangler.py
+++ b/src/atmst/mst/node_wrangler.py
@@ -37,7 +37,7 @@ def put_record(self, root_cid: CID, key: str, val: CID) -> CID:
 		root = self.ns.get_node(root_cid)
 		if root.is_empty(): # special case for empty tree
 			return self._put_here(root, key, val).cid
-		return self._put_recursive(root, key, val, MSTNode.key_height(key), root.height).cid
+		return self._put_recursive(root, key, val, MSTNode.key_height(key), root.definitely_height()).cid
 
 	def del_record(self, root_cid: CID, key: str) -> CID:
 		root = self.ns.get_node(root_cid)
@@ -45,7 +45,11 @@ def del_record(self, root_cid: CID, key: str) -> CID:
 		# Note: the seemingly redundant outer .get().cid is required to transform
 		# a None cid into the cid representing an empty node (we could maybe find a more elegant
 		# way of doing this...)
-		return self.ns.get_node(self._squash_top(self._delete_recursive(root, key, MSTNode.key_height(key), root.height))).cid
+		return self.ns.get_node(self._squash_top(
+			self._delete_recursive(
+				root, key, MSTNode.key_height(key), root.definitely_height()
+			)
+		)).cid
 
 
 
diff --git a/tests/test_mst_diff.py b/tests/test_mst_diff.py
index 31f71bf..e5d8545 100644
--- a/tests/test_mst_diff.py
+++ b/tests/test_mst_diff.py
@@ -43,8 +43,11 @@ def setUp(self):
 			self.trees.append(root)
 	
 	def test_diff_all_pairs(self):
-		for a in self.trees:
-			for b in self.trees:
+		for ai, a in enumerate(self.trees):
+			for bi, b in enumerate(self.trees):
+				#print(ai, bi)
+				#print(self.ns.pretty(a))
+				#print(self.ns.pretty(b))
 				reference_created, reference_deleted = very_slow_mst_diff(self.ns, a, b)
 				created, deleted = mst_diff(self.ns, a, b)
 				self.assertEqual(created, reference_created)