diff --git a/pysyncon/dataprep.py b/pysyncon/dataprep.py
index e951ec1..79b006d 100644
--- a/pysyncon/dataprep.py
+++ b/pysyncon/dataprep.py
@@ -283,9 +283,9 @@ def __str__(self) -> str:
         )
 
         if self.special_predictors:
-            str_special_pred = ''
+            str_special_pred = ""
             for predictor, time_range, op in self.special_predictors:
                 rep = f"    `{predictor}` over `{time_range}` using `{op}`\n"
                 str_special_pred = str_special_pred + rep
             str_rep = str_rep + f"Special predictors:\n" + str_special_pred
-        return str_rep
\ No newline at end of file
+        return str_rep