diff --git a/mypyc/build.py b/mypyc/build.py
index 8528aa088ac05..1cd6d109f0d0d 100644
--- a/mypyc/build.py
+++ b/mypyc/build.py
@@ -525,8 +525,9 @@ def mypycify(
     elif compiler.compiler_type == 'msvc':
         if opt_level == '3':
             opt_level = '2'
+        if opt_level != '0':
+            cflags.append('/O{}'.format(opt_level))
         cflags += [
-            '/O{}'.format(opt_level),
             '/wd4102',  # unreferenced label
             '/wd4101',  # unreferenced local variable
             '/wd4146',  # negating unsigned int
diff --git a/mypyc/test/test_run.py b/mypyc/test/test_run.py
index a5e356ea56a6e..d390a8a9501a4 100644
--- a/mypyc/test/test_run.py
+++ b/mypyc/test/test_run.py
@@ -263,7 +263,7 @@ def run_case_step(self, testcase: DataDrivenTestCase, incremental_step: int) ->
 
         # Assert that an output file got created
         suffix = 'pyd' if sys.platform == 'win32' else 'so'
-        assert glob.glob('native.*.{}'.format(suffix))
+        assert glob.glob('native.*.{}'.format(suffix)), list(os.listdir("."))
 
         driver_path = 'driver.py'
         if not os.path.isfile(driver_path):