diff --git a/machine_learning_hep/processer.py b/machine_learning_hep/processer.py
index 80a6e1bc29..9443e1e776 100644
--- a/machine_learning_hep/processer.py
+++ b/machine_learning_hep/processer.py
@@ -430,7 +430,8 @@ def dfuse(df_spec):
                         if not is_numeric_dtype(dfs[base][on]):
                             self.logger.info('exploding dataframe %s on variable %s', base, on)
                             dfs[out] = dfmerge(
-                                dfs[base].explode(on), dfs[ref], left_on=['df', on], suffixes=(f'_{base}', None), right_index=True)
+                                dfs[base].explode(on), dfs[ref], left_on=['df', on], suffixes=(f'_{base}', None),
+                                right_index=True)
                         else:
                             dfs[out] = dfmerge(
                                 dfs[base], dfs[ref], left_on=['df', on], suffixes=(f'_{base}', None), right_index=True)