diff --git a/synapse/storage/stats.py b/synapse/storage/stats.py
index 4925f5b62b..adb66a1340 100644
--- a/synapse/storage/stats.py
+++ b/synapse/storage/stats.py
@@ -195,7 +195,7 @@ class StatsStore(StateDeltasStore):
                     "table": table,
                     "id_col": id_col,
                     "origin_table": origin_table,
-                    "zero_cols": zero_cols,
+                    "zero_cols": ", ".join(zero_cols),
                     "zeroes": ", ".join(["0"] * len(zero_cols)),
                 }
             )