diff --git a/netflow/v9.py b/netflow/v9.py
index 4723313..adfd169 100644
--- a/netflow/v9.py
+++ b/netflow/v9.py
@@ -222,7 +222,7 @@ def __init__(self, data, template):
             elif flen == 1:
                 struct_format += 'B'
             else:
-                struct_format += f'{flen}s'
+                struct_format += '%ds' % flen
             struct_len += flen
 
         while offset <= (self.length - padding_size):