diff --git a/nixos/modules/config/networking.nix b/nixos/modules/config/networking.nix
index c09588834cf..ef747774b25 100644
--- a/nixos/modules/config/networking.nix
+++ b/nixos/modules/config/networking.nix
@@ -58,6 +58,7 @@ in
         "2.nixos.pool.ntp.org"
         "3.nixos.pool.ntp.org"
       ];
+      type = types.listOf types.str;
       description = ''
         The set of NTP servers from which to synchronise.
       '';
diff --git a/nixos/modules/services/networking/ntp/chrony.nix b/nixos/modules/services/networking/ntp/chrony.nix
index e6fa48daf46..115340924a2 100644
--- a/nixos/modules/services/networking/ntp/chrony.nix
+++ b/nixos/modules/services/networking/ntp/chrony.nix
@@ -41,6 +41,7 @@ in
 
       servers = mkOption {
         default = config.networking.timeServers;
+        type = types.listOf types.str;
         description = ''
           The set of NTP servers from which to synchronise.
         '';
diff --git a/nixos/modules/services/networking/ntp/ntpd.nix b/nixos/modules/services/networking/ntp/ntpd.nix
index 51398851adc..861b0db01a4 100644
--- a/nixos/modules/services/networking/ntp/ntpd.nix
+++ b/nixos/modules/services/networking/ntp/ntpd.nix
@@ -79,6 +79,7 @@ in
 
       servers = mkOption {
         default = config.networking.timeServers;
+        type = types.listOf types.str;
         description = ''
           The set of NTP servers from which to synchronise.
         '';
diff --git a/nixos/modules/system/boot/timesyncd.nix b/nixos/modules/system/boot/timesyncd.nix
index 35fb5578b07..692315dbe99 100644
--- a/nixos/modules/system/boot/timesyncd.nix
+++ b/nixos/modules/system/boot/timesyncd.nix
@@ -16,6 +16,7 @@ with lib;
       };
       servers = mkOption {
         default = config.networking.timeServers;
+        type = types.listOf types.str;
         description = ''
           The set of NTP servers from which to synchronise.
         '';