diff --git a/modules/services/networking/avahi-daemon.nix b/modules/services/networking/avahi-daemon.nix
index 791109dcda2b..3603d677837f 100644
--- a/modules/services/networking/avahi-daemon.nix
+++ b/modules/services/networking/avahi-daemon.nix
@@ -11,7 +11,13 @@ let
 
   avahiDaemonConf = with cfg; pkgs.writeText "avahi-daemon.conf" ''
     [server]
-    host-name=${hostName}
+    ${# Users can set `networking.hostName' to the empty string, when getting
+      # a host name from DHCP.  In that case, let Avahi take whatever the
+      # current host name is; setting `host-name' to the empty string in
+      # `avahi-daemon.conf' would be invalid.
+      if hostName != ""
+      then "host-name=${hostName}"
+      else ""}
     browse-domains=${concatStringsSep ", " browseDomains}
     use-ipv4=${if ipv4 then "yes" else "no"}
     use-ipv6=${if ipv6 then "yes" else "no"}