diff --git a/nixos/modules/tasks/network-interfaces.nix b/nixos/modules/tasks/network-interfaces.nix
index 75fc5a14a800..49901cda848d 100644
--- a/nixos/modules/tasks/network-interfaces.nix
+++ b/nixos/modules/tasks/network-interfaces.nix
@@ -417,7 +417,11 @@ in
         network node hostname (uname --nodename) the option
         boot.kernel.sysctl."kernel.hostname" can be used as a workaround (but
         the 64 character limit still applies).
+
+        WARNING: Do not use underscores (_) or you may run into unexpected issues.
       '';
+       # warning until the issues in https://github.com/NixOS/nixpkgs/pull/138978
+       # are resolved
     };
 
     networking.fqdn = mkOption {