diff --git a/testmon/testmon_core.py b/testmon/testmon_core.py
index 7d8b7e6..6603d87 100644
--- a/testmon/testmon_core.py
+++ b/testmon/testmon_core.py
@@ -90,7 +90,7 @@ def get_file(self, filename):
                     self.cache[filename] = Module(
                         source_code=code,
                         mtime=fs_mtime,
-                        ext=filename.rsplit(".", 1)[1],
+                        ext=filename.rsplit(".", 1)[1] if "." in filename else "",
                         fs_fsha=fsha,
                         filename=filename,
                         rootdir=self.rootdir,
diff --git a/tests/test_process_code.py b/tests/test_process_code.py
index 6238fcb..43bd887 100644
--- a/tests/test_process_code.py
+++ b/tests/test_process_code.py
@@ -497,3 +497,16 @@ def test_noncached_get_files_shas(testdir):
     testdir.run("git", "commit", "-m", "Initial commit")
 
     assert isinstance(noncached_get_files_shas(testdir.tmpdir.strpath), dict)
+
+
+def test_get_file_extensionless(tmp_path):
+    """Test that SourceTree.get_file handles files without extensions."""
+    extensionless = tmp_path / "Dockerfile"
+    extensionless.write_text("FROM python:3.10\n")
+
+    source_tree = SourceTree(str(tmp_path))
+    module = source_tree.get_file("Dockerfile")
+
+    assert module is not None
+    assert module.ext == ""
+    assert len(module.blocks) == 1