diff --git a/unit_test/main_options_test.py b/unit_test/main_options_test.py
index 1914f550..90935fe8 100644
--- a/unit_test/main_options_test.py
+++ b/unit_test/main_options_test.py
@@ -6,7 +6,7 @@ from cibuildwheel.__main__ import main
 from cibuildwheel.environment import ParsedEnvironment
 from cibuildwheel.util import BuildSelector
 
-from main_util_fixtures import mock_protection, platform, intercepted_build_args
+from main_util_fixtures import mock_protection, fake_project_dir, platform, intercepted_build_args
 
 
 
diff --git a/unit_test/main_platform_test.py b/unit_test/main_platform_test.py
index 101d3c4f..3de3be29 100644
--- a/unit_test/main_platform_test.py
+++ b/unit_test/main_platform_test.py
@@ -4,7 +4,7 @@ import sys
 
 from cibuildwheel.__main__ import main
 
-from main_util_fixtures import MOCK_PROJECT_DIR, mock_protection, platform, intercepted_build_args
+from main_util_fixtures import MOCK_PROJECT_DIR, mock_protection, fake_project_dir, platform, intercepted_build_args
 
 
 def test_unknown_platform_non_ci(monkeypatch, capsys):
diff --git a/unit_test/main_util_fixtures.py b/unit_test/main_util_fixtures.py
index ae7bd006..8bc80d87 100644
--- a/unit_test/main_util_fixtures.py
+++ b/unit_test/main_util_fixtures.py
@@ -17,6 +17,11 @@ MOCK_PROJECT_DIR = 'some_project_dir'
 
 @pytest.fixture(autouse=True)
 def mock_protection(monkeypatch):
+    '''
+    Ensure that a unit test will never actually run a cibuildwheel 'build'
+    function, which shouldn't be run on a developer's machine
+    '''
+
     def fail_on_call(*args, **kwargs):
         raise RuntimeError("This should never be called")
 
@@ -25,7 +30,21 @@ def mock_protection(monkeypatch):
     monkeypatch.setattr(windows, 'build', fail_on_call)
     monkeypatch.setattr(linux, 'build', fail_on_call)
     monkeypatch.setattr(macos, 'build', fail_on_call)
-    monkeypatch.setattr(os.path, 'exists', lambda x: True)
+
+@pytest.fixture(autouse=True)
+def fake_project_dir(monkeypatch):
+    '''
+    Monkey-patch enough for the main() function to run
+    '''
+
+    real_os_path_exists = os.path.exists
+    def mock_os_path_exists(path):
+        if path == os.path.join(MOCK_PROJECT_DIR, 'setup.py'):
+            return True
+        else:
+            return real_os_path_exists(path)
+
+    monkeypatch.setattr(os.path, 'exists', mock_os_path_exists)
     monkeypatch.setattr(sys, 'argv', ['cibuildwheel', MOCK_PROJECT_DIR])