diff --git a/nix/pkgs/subnetplan/render.rb b/nix/pkgs/subnetplan/render.rb
index c41ef11..aa74757 100644
--- a/nix/pkgs/subnetplan/render.rb
+++ b/nix/pkgs/subnetplan/render.rb
@@ -63,7 +63,7 @@ nets.each do |net|
   (groups[group] ||= []) << net
 end
 
-max_prefix = groups.collect { nets.collect { |net| net.addr.prefix }.max }.max
+max_prefix = [groups.collect { nets.collect { |net| net.addr.prefix }.max }.max, 64].min
 groups = groups.collect do |group, nets|
   allnet = nets[0].addr.clone
   while allnet.prefix > 0 and not allnet.include?(nets[nets.size - 1].addr)