diff --git a/dpbench/infrastructure/reporter.py b/dpbench/infrastructure/reporter.py
index 693f3d8d..68f45693 100644
--- a/dpbench/infrastructure/reporter.py
+++ b/dpbench/infrastructure/reporter.py
@@ -199,10 +199,7 @@ def generate_performance_report(
         .where(dm.Result.run_id == run_id)
     )
 
-    df = pd.read_sql_query(
-        sql=sql,
-        con=conn.connect(),
-    )
+    df = pd.read_sql_query(sql=sql, con=conn.connect()).astype("string")
 
     for index, row in df.iterrows():
         for impl in implementations: