diff --git a/src/buildstream_plugins/sources/git.py b/src/buildstream_plugins/sources/git.py
index 0643947..06ec92a 100644
--- a/src/buildstream_plugins/sources/git.py
+++ b/src/buildstream_plugins/sources/git.py
@@ -1076,7 +1076,11 @@ def _guess_version(self):
         commit_sha = None
 
         string = self.mirror.ref
-
+        if string is None:
+            # An untracked source has no ref to guess from: the "sha"
+            # cannot be discovered, so honor the documented contract
+            # ("either None or strings") instead of crashing on None.
+            return (None, version_guess, commits)
         main_split = string.rsplit("-g", 1)
         if len(main_split) < 2:
             # Didn't find the `-g`, assume its a raw git sha as input
@@ -1097,6 +1101,8 @@ def _guess_version(self):
         return (commit_sha, version_guess, commits)
 
     def collect_source_info(self):
+        if self.mirror.ref is None:
+            return []
         extra_data = None
         commit_sha, version_guess, commits = self._guess_version()
         if commits:
diff --git a/tests/sources/git.py b/tests/sources/git.py
index 5b766a3..2983dc2 100644
--- a/tests/sources/git.py
+++ b/tests/sources/git.py
@@ -1243,3 +1243,22 @@ def test_source_info(
         commit_offset = None
 
     assert commit_offset == expected_offset
+
+
+@pytest.mark.skipif(HAVE_GIT is False, reason="git is not available")
+@pytest.mark.datafiles(os.path.join(DATA_DIR, "template"))
+def test_source_info_untracked_source(cli, tmpdir, datafiles):
+    project = str(datafiles)
+
+    # Create the repo, but never track the element: the source
+    # is valid (has a track) yet has no ref.
+    repo = create_repo("git", str(tmpdir))
+    repo.create(os.path.join(project, "repofiles"))
+
+    # Write out our test target with track and NO ref
+    element = {"kind": "import", "sources": [repo.source_config(ref=None)]}
+    generate_element(project, "target.bst", element)
+
+    # `bst show` with %{source-info} must not crash on the untracked source
+    result = cli.run(project=project, args=["show", "--format", "%{name}:\n%{source-info}", "target.bst"])
+    result.assert_success()
\ No newline at end of file