diff --git a/mail-server/dovecot.nix b/mail-server/dovecot.nix
index dc51c61..b420b0d 100644
--- a/mail-server/dovecot.nix
+++ b/mail-server/dovecot.nix
@@ -398,9 +398,7 @@ in
             fields = {
               home = "${cfg.storage.path}/%{user | domain}/%{user | username}";
               inherit (cfg.storage) uid gid;
-              mail_index_path = "${
-                if cfg.indexDir != null then cfg.indexDir else cfg.storage.path
-              }/%{user | domain }/%{user | username}";
+              mail_index_path = mkIf (cfg.indexDir != null) "${cfg.indexDir}/%{user | domain}/%{user | username}";
             };
           };
           "passdb declarative" = {
@@ -428,9 +426,9 @@ in
             fields = {
               home = "${cfg.storage.path}/ldap/%{ldap:${cfg.ldap.attributes.uuid}}";
               inherit (cfg.storage) uid gid;
-              mail_index_path = "${
-                if cfg.indexDir != null then cfg.indexDir else cfg.storage.path
-              }/ldap/%{ldap:${cfg.ldap.attributes.uuid}}";
+              mail_index_path = mkIf (
+                cfg.indexDir != null
+              ) "${cfg.indexDir}/ldap/%{ldap:${cfg.ldap.attributes.uuid}}";
             };
             ldap_connection_group = "ldap-userdb-conn";
           };