diff --git a/change_quality.py b/change_quality.py
index 9d5f194..b5fc4a4 100644
--- a/change_quality.py
+++ b/change_quality.py
@@ -39,6 +39,7 @@ def change_quality_fw(input_file, output_file):
             elif line_num % 4 == 2:
                 f_out.write(line)
             else:
+                read_length = len(line.strip())
                 new_quality = (
                     generate_quality_first_bases_fw(read_length) +
                     generate_quality_next_bases_fw(read_length) +