diff --git a/optim_esm_tools/analyze/combine_variables.py b/optim_esm_tools/analyze/combine_variables.py
index 6f555b0..4e34793 100644
--- a/optim_esm_tools/analyze/combine_variables.py
+++ b/optim_esm_tools/analyze/combine_variables.py
@@ -42,10 +42,10 @@ def __init__(
         self.tipping_thresholds = tipping_thresholds
         self.table_formats = table_formats
         if data_set:
-            self.source_files = dict(
+            self.source_files = dict(zip(
                 oet.utils.to_str_tuple(data_set.attrs['variables']),
                 oet.utils.to_str_tuple(data_set.attrs['source_files']),
-            )
+            ))
             self.common_mask = data_set['global_mask']
             return  # pragma: no cover
         source_files, common_mask = self.process_masks()