diff --git a/src/dtscalibration/datastore.py b/src/dtscalibration/datastore.py
index 1c32e781..806a7a01 100644
--- a/src/dtscalibration/datastore.py
+++ b/src/dtscalibration/datastore.py
@@ -2386,7 +2386,7 @@ def calibration_double_ended(
 
         if method == "wls":
             p_cov, p_val, p_var = calibration_double_ended_helper(
-                self
+                self,
                 st_var,
                 ast_var,
                 rst_var,