diff --git a/pyhdx/web/controllers.py b/pyhdx/web/controllers.py
index 47ff446a..1084379b 100644
--- a/pyhdx/web/controllers.py
+++ b/pyhdx/web/controllers.py
@@ -1557,7 +1557,7 @@ def add_drfu_comparison(self):
         names = ["comparison_name", "comparison_state", "exposure", "quantity"]
 
         # Take rfu entries from df, to calculate drfu
-        reference_rfu = rfu_df.xs(key=[self.reference_state, "rfu"], level=[0, 2], axis=1)
+        reference_rfu = rfu_df.xs(key=(self.reference_state, "rfu"), level=[0, 2], axis=1)
         test_rfu = rfu_df.drop(self.reference_state, axis=1, level=0).xs("rfu", level=2, axis=1)
 
         drfu = test_rfu.sub(reference_rfu, level="exposure").dropna(how="all", axis=1)