diff --git a/src/main/scala/gitbucket/core/controller/AccountController.scala b/src/main/scala/gitbucket/core/controller/AccountController.scala
index f2959ca..04455ea 100644
--- a/src/main/scala/gitbucket/core/controller/AccountController.scala
+++ b/src/main/scala/gitbucket/core/controller/AccountController.scala
@@ -139,7 +139,7 @@
   private def accountWebhookEvents = new ValueType[Set[WebHook.Event]]{
     def convert(name: String, params: Map[String, Seq[String]], messages: Messages): Set[WebHook.Event] = {
       WebHook.Event.values.flatMap { t =>
-        params.get(name + "." + t.name).map(_ => t)
+        params.optionValue(name + "." + t.name).map(_ => t)
       }.toSet
     }
     def validate(name: String, params: Map[String, Seq[String]], messages: Messages): Seq[(String, String)] =
@@ -636,10 +636,14 @@
   }
 
   private def uniqueRepository: Constraint = new Constraint(){
-    override def validate(name: String, value: String, params: Map[String, Seq[String]], messages: Messages): Option[String] =
-      params.get("owner").flatMap { userName =>
-        getRepositoryNamesOfUser(userName.head).find(_ == value).map(_ => "Repository already exists.")
+    override def validate(name: String, value: String, params: Map[String, Seq[String]], messages: Messages): Option[String] = {
+      for {
+        userName <- params.optionValue("owner")
+        _        <- getRepositoryNamesOfUser(userName).find(_ == value)
+      } yield {
+        "Repository already exists."
       }
+    }
   }
 
   private def members: Constraint = new Constraint(){
diff --git a/src/main/scala/gitbucket/core/controller/ControllerBase.scala b/src/main/scala/gitbucket/core/controller/ControllerBase.scala
index 4948c43..993f160 100644
--- a/src/main/scala/gitbucket/core/controller/ControllerBase.scala
+++ b/src/main/scala/gitbucket/core/controller/ControllerBase.scala
@@ -314,10 +314,11 @@
   }
 
   protected def uniqueMailAddress(paramName: String = ""): Constraint = new Constraint(){
-    override def validate(name: String, value: String, params: Map[String, Seq[String]], messages: Messages): Option[String] =
+    override def validate(name: String, value: String, params: Map[String, Seq[String]], messages: Messages): Option[String] = {
       getAccountByMailAddress(value, true)
-        .filter { x => if(paramName.isEmpty) true else Some(x.userName) != params.get(paramName) }
+        .filter { x => if(paramName.isEmpty) true else Some(x.userName) != params.optionValue(paramName) }
         .map    { _ => "Mail address is already registered." }
+    }
   }
 
   val allReservedNames = Set("git", "admin", "upload", "api", "assets", "plugin-assets", "signin", "signout", "register", "activities.atom", "sidebar-collapse", "groups", "new")
diff --git a/src/main/scala/gitbucket/core/controller/LabelsController.scala b/src/main/scala/gitbucket/core/controller/LabelsController.scala
index d0d82d4..4f42475 100644
--- a/src/main/scala/gitbucket/core/controller/LabelsController.scala
+++ b/src/main/scala/gitbucket/core/controller/LabelsController.scala
@@ -4,6 +4,7 @@
 import gitbucket.core.service.{RepositoryService, AccountService, IssuesService, LabelsService}
 import gitbucket.core.util.{ReferrerAuthenticator, WritableUsersAuthenticator}
 import gitbucket.core.util.Implicits._
+import gitbucket.core.util.SyntaxSugars._
 import org.scalatra.forms._
 import org.scalatra.i18n.Messages
 import org.scalatra.Ok
@@ -83,10 +84,10 @@
 
   private def uniqueLabelName: Constraint = new Constraint(){
     override def validate(name: String, value: String, params: Map[String, Seq[String]], messages: Messages): Option[String] = {
-      val owner = params("owner").head
-      val repository = params("repository").head
-      params.get("labelId").map { labelId =>
-        getLabel(owner, repository, value).filter(_.labelId != labelId.head.toInt).map(_ => "Name has already been taken.")
+      val owner      = params.value("owner")
+      val repository = params.value("repository")
+      params.optionValue("labelId").map { labelId =>
+        getLabel(owner, repository, value).filter(_.labelId != labelId.toInt).map(_ => "Name has already been taken.")
       }.getOrElse {
         getLabel(owner, repository, value).map(_ => "Name has already been taken.")
       }
diff --git a/src/main/scala/gitbucket/core/controller/PrioritiesController.scala b/src/main/scala/gitbucket/core/controller/PrioritiesController.scala
index 340065f..dee6cbb 100644
--- a/src/main/scala/gitbucket/core/controller/PrioritiesController.scala
+++ b/src/main/scala/gitbucket/core/controller/PrioritiesController.scala
@@ -4,6 +4,7 @@
 import gitbucket.core.service.{RepositoryService, AccountService, IssuesService, PrioritiesService}
 import gitbucket.core.util.{ReferrerAuthenticator, WritableUsersAuthenticator}
 import gitbucket.core.util.Implicits._
+import gitbucket.core.util.SyntaxSugars._
 import org.scalatra.forms._
 import org.scalatra.i18n.Messages
 import org.scalatra.Ok
@@ -99,10 +100,10 @@
 
   private def uniquePriorityName: Constraint = new Constraint(){
     override def validate(name: String, value: String, params: Map[String, Seq[String]], messages: Messages): Option[String] = {
-      val owner = params("owner").head
-      val repository = params("repository").head
-      params.get("priorityId").map { priorityId =>
-        getPriority(owner, repository, value).filter(_.priorityId != priorityId.head.toInt).map(_ => "Name has already been taken.")
+      val owner      = params.value("owner")
+      val repository = params.value("repository")
+      params.optionValue("priorityId").map { priorityId =>
+        getPriority(owner, repository, value).filter(_.priorityId != priorityId.toInt).map(_ => "Name has already been taken.")
       }.getOrElse {
         getPriority(owner, repository, value).map(_ => "Name has already been taken.")
       }
diff --git a/src/main/scala/gitbucket/core/controller/RepositorySettingsController.scala b/src/main/scala/gitbucket/core/controller/RepositorySettingsController.scala
index cab7c07..9e6c0b0 100644
--- a/src/main/scala/gitbucket/core/controller/RepositorySettingsController.scala
+++ b/src/main/scala/gitbucket/core/controller/RepositorySettingsController.scala
@@ -460,12 +460,15 @@
    * Duplicate check for the rename repository name.
    */
   private def renameRepositoryName: Constraint = new Constraint(){
-    override def validate(name: String, value: String, params: Map[String, Seq[String]], messages: Messages): Option[String] =
-      params.get("repository").filter(_ != value).flatMap { _ =>
-        params.get("owner").flatMap { userName =>
-          getRepositoryNamesOfUser(userName.head).find(_ == value).map(_ => "Repository already exists.")
-        }
+    override def validate(name: String, value: String, params: Map[String, Seq[String]], messages: Messages): Option[String] = {
+      for {
+        repoName <- params.optionValue("repository") if repoName != value
+        userName <- params.optionValue("owner")
+        _        <- getRepositoryNamesOfUser(userName).find(_ == value)
+      } yield {
+        "Repository already exists."
       }
+    }
   }
 
   /**
diff --git a/src/main/scala/gitbucket/core/controller/WikiController.scala b/src/main/scala/gitbucket/core/controller/WikiController.scala
index b212b81..5dde40e 100644
--- a/src/main/scala/gitbucket/core/controller/WikiController.scala
+++ b/src/main/scala/gitbucket/core/controller/WikiController.scala
@@ -227,7 +227,7 @@
 
   private def unique: Constraint = new Constraint(){
     override def validate(name: String, value: String, params: Map[String, Seq[String]], messages: Messages): Option[String] =
-      getWikiPageList(params("owner").head, params("repository").head).find(_ == value).map(_ => "Page already exists.")
+      getWikiPageList(params.value("owner"), params.value("repository")).find(_ == value).map(_ => "Page already exists.")
   }
 
   private def pagename: Constraint = new Constraint(){
diff --git a/src/main/scala/gitbucket/core/util/SyntaxSugars.scala b/src/main/scala/gitbucket/core/util/SyntaxSugars.scala
index 1c0384a..74d39cb 100644
--- a/src/main/scala/gitbucket/core/util/SyntaxSugars.scala
+++ b/src/main/scala/gitbucket/core/util/SyntaxSugars.scala
@@ -53,4 +53,14 @@
     def unapply[A, B](t: (A, B)): Option[(A, B)] = Some(t)
   }
 
+  /**
+   * Provides easier and explicit ways to access to a head value of `Map[String, Seq[String]]`.
+   * This is intended to use in implementations of scalatra-forms's `Constraint` or `ValueType`.
+   */
+  implicit class HeadValueAccessibleMap(map: Map[String, Seq[String]]){
+    def value(key: String): String = map(key).head
+    def optionValue(key: String): Option[String] = map.get(key).flatMap(_.headOption)
+    def values(key: String): Seq[String] = map.get(key).getOrElse(Seq.empty)
+  }
+
 }