diff --git a/earwigbot/commands/cidr.py b/earwigbot/commands/cidr.py
index a4e285e..c424bc3 100644
--- a/earwigbot/commands/cidr.py
+++ b/earwigbot/commands/cidr.py
@@ -129,7 +129,7 @@ class CIDR(Command):
         bin_ips = ["".join(
             bin(ord(octet))[2:].zfill(8) for octet in ip.ip) for ip in ips]
         for i, ip in enumerate(ips):
-            if ip.size:
+            if ip.size is not None:
                 suffix = "X" * (len(bin_ips[i]) - ip.size)
                 bin_ips[i] = bin_ips[i][:ip.size] + suffix