diff --git a/tagbot/action/git.py b/tagbot/action/git.py
index 04c9a5b3..b0fae6c2 100644
--- a/tagbot/action/git.py
+++ b/tagbot/action/git.py
@@ -70,6 +70,16 @@ def check(self, *argv: str, repo: Optional[str] = "") -> bool:
         except Abort:
             return False
 
+    def commit_sha_of_tree(self, tree: str) -> Optional[str]:
+        """Get the commit SHA of a corresponding tree SHA."""
+        # We need --all in case the registered commit isn't on the default branch.
+        for line in self.command("log", "--all", "--format=%H %T").splitlines():
+            # The format of each line is "<commit sha> <tree sha>".
+            c, t = line.split()
+            if t == tree:
+                return c
+        return None
+
     def commit_sha_of_default(self) -> str:
         """Get the commit SHA of the default branch."""
         return self.command("rev-parse", self._default_branch)
diff --git a/tagbot/action/repo.py b/tagbot/action/repo.py
index aee0f5df..29f4e130 100644
--- a/tagbot/action/repo.py
+++ b/tagbot/action/repo.py
@@ -122,7 +122,10 @@ def _commit_sha_of_tree(self, tree: str) -> Optional[str]:
             sha = self._commit_sha_of_tree_from_branch(branch.name, tree, since)
             if sha:
                 return sha
-        return None
+        # For a valid tree SHA, the only time that we reach here is when a release
+        # has been made long after the commit was made, which is reasonably rare.
+        # Fall back to cloning the repo in that case.
+        return self._git.commit_sha_of_tree(tree)
 
     def _commit_sha_of_tag(self, version: str) -> Optional[str]:
         """Look up the commit SHA of a given tag."""
diff --git a/test/action/test_git.py b/test/action/test_git.py
index ff1c74ab..05ad5251 100644
--- a/test/action/test_git.py
+++ b/test/action/test_git.py
@@ -77,6 +77,14 @@ def test_default_branch():
     assert g._default_branch == "master"
 
 
+def test_commit_sha_of_tree():
+    g = _git(command="a b\n c d\n d e\n")
+    assert g.commit_sha_of_tree("b") == "a"
+    g.command.assert_called_with("log", "--all", "--format=%H %T")
+    assert g.commit_sha_of_tree("e") == "d"
+    assert g.commit_sha_of_tree("c") is None
+
+
 def test_commit_sha_of_default():
     g = _git(command="abcdef")
     g._Git__default_branch = "branch"
diff --git a/test/action/test_repo.py b/test/action/test_repo.py
index 7397b3dc..5db0f9b3 100644
--- a/test/action/test_repo.py
+++ b/test/action/test_repo.py
@@ -106,6 +106,11 @@ def test_commit_sha_of_tree():
     r._commit_sha_of_tree_from_branch.assert_called_once_with("master", "tree", now)
     assert r._commit_sha_of_tree("tree") == "sha2"
     r._commit_sha_of_tree_from_branch.assert_called_with("foo", "tree", now)
+    r._commit_sha_of_tree_from_branch.side_effect = None
+    r._commit_sha_of_tree_from_branch.return_value = None
+    r._git.commit_sha_of_tree = Mock(side_effect=["sha", None])
+    assert r._commit_sha_of_tree("tree") == "sha"
+    assert r._commit_sha_of_tree("tree") is None
 
 
 def test_commit_sha_of_tag():