diff --git a/src/coverup/coverup.py b/src/coverup/coverup.py
index 06d26a9..8955237 100644
--- a/src/coverup/coverup.py
+++ b/src/coverup/coverup.py
@@ -156,6 +156,19 @@ def positive_int(value):
     if not args.model:
         ap.error('Specify the model to use with --model')
 
+    if not list(args.module_dir.glob("*.py")):
+        sources = sorted(args.module_dir.glob("**/*.py"), key=lambda p: len(p.parts))
+        suggestion = sources[0].parent if sources else None
+
+        try:
+            args.module_dir = args.module_dir.relative_to(Path.cwd())
+            suggestion = suggestion.relative_to(Path.cwd()) if suggestion else None
+        except ValueError:
+            pass
+
+        ap.error(f'No Python sources found in "{args.module_dir}"' +
+                 (f'; did you mean "{suggestion}"?' if suggestion else '.'))
+
     return args
 
 
diff --git a/tests/test_coverup_25.py b/tests/test_coverup_25.py
index f38fb1d..fc22d18 100644
--- a/tests/test_coverup_25.py
+++ b/tests/test_coverup_25.py
@@ -13,5 +13,6 @@ def test_parse_args_with_source_files(tmp_path):
     tests_dir.mkdir()
     source_dir = tmp_path / "source"
     source_dir.mkdir()
+    (source_dir / "m.py").touch()
     args = parse_args([str(source_file), '--tests-dir', str(tests_dir), '--source-dir', str(source_dir), '--model', 'gpt-4'])
     assert args.source_files[0] == source_file.resolve()
diff --git a/tests/test_prompt.py b/tests/test_prompt.py
index 82ea33e..189167c 100644
--- a/tests/test_prompt.py
+++ b/tests/test_prompt.py
@@ -15,6 +15,7 @@ def test_gpt4_v1_relative_file_name(tmp_path, monkeypatch):
     monkeypatch.chdir(tmp_path)
 
     (tmp_path / "lib" / "ansible").mkdir(parents=True)
+    (tmp_path / "lib" / "ansible" / "__init__.py").touch()
     (tmp_path / "tests").mkdir(parents=True)
 
     from coverup.coverup import parse_args
@@ -42,6 +43,7 @@ def test_claude_relative_file_name(tmp_path, monkeypatch):
     monkeypatch.chdir(tmp_path)
 
     (tmp_path / "lib" / "ansible").mkdir(parents=True)
+    (tmp_path / "lib" / "ansible" / "__init__.py").touch()
     (tmp_path / "tests").mkdir(parents=True)
 
     from coverup.coverup import parse_args