diff --git a/pytest_cpp/boost.py b/pytest_cpp/boost.py
index a436efd..649878d 100644
--- a/pytest_cpp/boost.py
+++ b/pytest_cpp/boost.py
@@ -42,7 +42,7 @@ def read_file(name):
         report_xml = os.path.join(temp_dir, 'report.xml')
         args = [
             executable,
-            '--output_format=xml',
+            '--output_format=XML',
             '--log_sink=%s' % log_xml,
             '--report_sink=%s' % report_xml,
         ]