diff --git a/bin/chunk_assembly_for_vecscreen.py b/bin/chunk_assembly_for_vecscreen.py
index 7f08e54..38bce8d 100755
--- a/bin/chunk_assembly_for_vecscreen.py
+++ b/bin/chunk_assembly_for_vecscreen.py
@@ -20,7 +20,7 @@ def process_record(record, chunk_num, threshold_length, overlap_length):
 def generate_records_to_write(record, threshold_length, overlap_length):
     return [
         (
-            record_slice.id + f".chunk_{chunk_num + 1}",
+            record_slice.id + ".chunk_{}".format(chunk_num + 1),
             "",
             record_slice,
         )