diff --git a/pkgs/zerotier-members/zerotier-members.py b/pkgs/zerotier-members/zerotier-members.py
index d69642862..322a6fc96 100755
--- a/pkgs/zerotier-members/zerotier-members.py
+++ b/pkgs/zerotier-members/zerotier-members.py
@@ -15,7 +15,7 @@ class ClanError(Exception):
 
 def compute_zerotier_ip(network_id: str, identity: str) -> ipaddress.IPv6Address:
     assert len(network_id) == 16, (
-        "network_id must be 16 characters long, got {network_id}"
+        f"network_id must be 16 characters long, got {network_id}"
     )
     nwid = int(network_id, 16)
     node_id = int(identity, 16)