diff --git a/cobra/preprocessing/preprocessor.py b/cobra/preprocessing/preprocessor.py
index 3cd94c8..0177e34 100644
--- a/cobra/preprocessing/preprocessor.py
+++ b/cobra/preprocessing/preprocessor.py
@@ -227,6 +227,9 @@ def fit(self, train_data: pd.DataFrame, continuous_vars: list,
         log.info("Starting to fit pipeline")
         start = time.time()
 
+        # Ensure to operate on separate copy of data
+        train_data = train_data.copy()
+
         # Fit discretizer, categorical preprocessor & target encoder
         # Note that in order to fit target_encoder, we first have to transform
         # the data using the fitted discretizer & categorical_data_processor