diff --git a/nixos/modules/security/duosec.nix b/nixos/modules/security/duosec.nix
index 71428b82f5da..c47be80b9dc3 100644
--- a/nixos/modules/security/duosec.nix
+++ b/nixos/modules/security/duosec.nix
@@ -51,7 +51,7 @@ in
       };
 
       secretKeyFile = mkOption {
-        type = types.path;
+        type = types.nullOr types.path;
         default = null;
         description = ''
           A file containing your secret key. The security of your Duo application is tied to the security of your secret key.
diff --git a/nixos/modules/services/audio/icecast.nix b/nixos/modules/services/audio/icecast.nix
index f40ea6be29d4..6ca20a7a1086 100644
--- a/nixos/modules/services/audio/icecast.nix
+++ b/nixos/modules/services/audio/icecast.nix
@@ -47,7 +47,7 @@ in {
       enable = mkEnableOption "Icecast server";
 
       hostname = mkOption {
-        type = types.str;
+        type = types.nullOr types.str;
         description = "DNS name or IP address that will be used for the stream directory lookups or possibily the playlist generation if a Host header is not provided.";
         default = config.networking.domain;
       };
diff --git a/nixos/modules/services/hardware/fancontrol.nix b/nixos/modules/services/hardware/fancontrol.nix
index bb4541a784da..e1ce11a5aef6 100644
--- a/nixos/modules/services/hardware/fancontrol.nix
+++ b/nixos/modules/services/hardware/fancontrol.nix
@@ -12,7 +12,7 @@ in{
 
     config = mkOption {
       default = null;
-      type = types.lines;
+      type = types.nullOr types.lines;
       description = "Fancontrol configuration file content. See <citerefentry><refentrytitle>pwmconfig</refentrytitle><manvolnum>8</manvolnum></citerefentry> from the lm_sensors package.";
       example = ''
         # Configuration file generated by pwmconfig
diff --git a/nixos/modules/services/misc/gammu-smsd.nix b/nixos/modules/services/misc/gammu-smsd.nix
index 3057d7fd1a09..552725f1384d 100644
--- a/nixos/modules/services/misc/gammu-smsd.nix
+++ b/nixos/modules/services/misc/gammu-smsd.nix
@@ -172,7 +172,7 @@ in {
           };
 
           database = mkOption {
-            type = types.str;
+            type = types.nullOr types.str;
             default = null;
             description = "Database name to store sms data";
           };
diff --git a/nixos/modules/services/network-filesystems/orangefs/server.nix b/nixos/modules/services/network-filesystems/orangefs/server.nix
index 74ebdc134024..8eb754fe6110 100644
--- a/nixos/modules/services/network-filesystems/orangefs/server.nix
+++ b/nixos/modules/services/network-filesystems/orangefs/server.nix
@@ -83,14 +83,14 @@ in {
       };
 
       dataStorageSpace = mkOption {
-        type = types.str;
+        type = types.nullOr types.str;
         default = null;
         example = "/data/storage";
         description = "Directory for data storage.";
       };
 
       metadataStorageSpace = mkOption {
-        type = types.str;
+        type = types.nullOr types.str;
         default = null;
         example = "/data/meta";
         description = "Directory for meta data storage.";
diff --git a/nixos/modules/services/networking/monero.nix b/nixos/modules/services/networking/monero.nix
index 97af29978397..fde3293fc131 100644
--- a/nixos/modules/services/networking/monero.nix
+++ b/nixos/modules/services/networking/monero.nix
@@ -87,7 +87,7 @@ in
       };
 
       rpc.password = mkOption {
-        type = types.str;
+        type = types.nullOr types.str;
         default = null;
         description = ''
           Password for RPC connections.
diff --git a/nixos/modules/services/networking/namecoind.nix b/nixos/modules/services/networking/namecoind.nix
index 6ca99e1321bd..16f85df2e77c 100644
--- a/nixos/modules/services/networking/namecoind.nix
+++ b/nixos/modules/services/networking/namecoind.nix
@@ -89,7 +89,7 @@ in
       };
 
       rpc.password = mkOption {
-        type = types.str;
+        type = types.nullOr types.str;
         default = null;
         description = ''
           Password for RPC connections.
diff --git a/nixos/modules/system/boot/luksroot.nix b/nixos/modules/system/boot/luksroot.nix
index 166f89c70661..e6a53b03f88d 100644
--- a/nixos/modules/system/boot/luksroot.nix
+++ b/nixos/modules/system/boot/luksroot.nix
@@ -641,7 +641,7 @@ in
             credential = mkOption {
               default = null;
               example = "f1d00200d8dc783f7fb1e10ace8da27f8312d72692abfca2f7e4960a73f48e82e1f7571f6ebfcee9fb434f9886ccc8fcc52a6614d8d2";
-              type = types.str;
+              type = types.nullOr types.str;
               description = "The FIDO2 credential ID.";
             };
 
diff --git a/nixos/modules/virtualisation/parallels-guest.nix b/nixos/modules/virtualisation/parallels-guest.nix
index 828419fb4b9d..55605b388b7c 100644
--- a/nixos/modules/virtualisation/parallels-guest.nix
+++ b/nixos/modules/virtualisation/parallels-guest.nix
@@ -32,7 +32,7 @@ in
       };
 
       package = mkOption {
-        type = types.package;
+        type = types.nullOr types.package;
         default = config.boot.kernelPackages.prl-tools;
         defaultText = "config.boot.kernelPackages.prl-tools";
         example = literalExample "config.boot.kernelPackages.prl-tools";