diff --git a/cello/cello_predict.py b/cello/cello_predict.py
index 39e5f21..bcb5894 100644
--- a/cello/cello_predict.py
+++ b/cello/cello_predict.py
@@ -189,7 +189,7 @@ def main():
             for x in finalized_binary_results_df.columns
         ]
         ms_results_df['most_specific_cell_type'] = [
-            ou.cell_ontology().id_to_term[x].name
+            ou.cell_ontology().id_to_term[x].name if x != '' else 'Unknown'
             for x in ms_results_df['most_specific_cell_type']
         ]