diff --git a/lit-test/test/lit.cfg b/lit-test/test/lit.cfg
index 21b0acc09..0a5cb2c74 100644
--- a/lit-test/test/lit.cfg
+++ b/lit-test/test/lit.cfg
@@ -14,7 +14,7 @@ python_executable = 'python3.5'
 
 harness = os.path.join(root, 'harness.py')
 if os.getenv('COVERAGE'):
-    harness = 'coverage run --append --source=artiq {}'.format(harness)
+    harness = 'coverage run --parallel-mode --source=artiq {}'.format(harness)
     config.environment['COVERAGE_FILE'] = os.path.join(root, '..', '.coverage')
     config.substitutions.append( ('%python', harness) )
 else: