diff --git a/test_runner/fixtures/benchmark_fixture.py b/test_runner/fixtures/benchmark_fixture.py
index a2a8cc64d3..328ebcc1f8 100644
--- a/test_runner/fixtures/benchmark_fixture.py
+++ b/test_runner/fixtures/benchmark_fixture.py
@@ -151,7 +151,7 @@ class ZenithBenchmarker:
                             re.MULTILINE)
         return int(round(float(matches.group(1))))
 
-    def get_timeline_size(repo_dir: str, tenantid: str, timelineid: str):
+    def get_timeline_size(self, repo_dir: str, tenantid: str, timelineid: str):
         """
         Calculate the on-disk size of a timeline
         """