diff --git a/mercurial/manifest.py b/mercurial/manifest.py
--- a/mercurial/manifest.py
+++ b/mercurial/manifest.py
@@ -760,6 +760,7 @@ def _splittopdir(f):
 _noop = lambda s: None
 
 
+@interfaceutil.implementer(repository.imanifestdict)
 class treemanifest(object):
     def __init__(self, dir=b'', text=b''):
         self._dir = dir
diff --git a/tests/test-check-interfaces.py b/tests/test-check-interfaces.py
--- a/tests/test-check-interfaces.py
+++ b/tests/test-check-interfaces.py
@@ -211,6 +211,7 @@ def main():
 
     ziverify.verifyClass(repository.ifilestorage, filelog.filelog)
     ziverify.verifyClass(repository.imanifestdict, manifest.manifestdict)
+    ziverify.verifyClass(repository.imanifestdict, manifest.treemanifest)
     ziverify.verifyClass(
         repository.imanifestrevisionstored, manifest.manifestctx
     )