diff --git a/akka-http-core/src/main/scala/akka/http/model/HttpMessage.scala b/akka-http-core/src/main/scala/akka/http/model/HttpMessage.scala
index 8988c401ee..daa097dde4 100644
--- a/akka-http-core/src/main/scala/akka/http/model/HttpMessage.scala
+++ b/akka-http-core/src/main/scala/akka/http/model/HttpMessage.scala
@@ -31,7 +31,7 @@ sealed trait HttpMessage extends japi.HttpMessage {
   def protocol: HttpProtocol
 
   /** Returns a copy of this message with the list of headers set to the given ones. */
-  def withHeaders(headers: HttpHeader*): Self = withHeaders(immutable.Seq(headers: _*))
+  def withHeaders(headers: HttpHeader*): Self = withHeaders(headers.toList)
 
   /** Returns a copy of this message with the list of headers set to the given ones. */
   def withHeaders(headers: immutable.Seq[HttpHeader]): Self
@@ -40,9 +40,16 @@ sealed trait HttpMessage extends japi.HttpMessage {
    * Returns a new message that contains all of the given default headers which didn't already
    * exist (by case-insensitive header name) in this message.
    */
-  def withDefaultHeaders(defaultHeaders: immutable.Seq[HttpHeader]) =
+  def withDefaultHeaders(defaultHeaders: HttpHeader*): Self = withDefaultHeaders(defaultHeaders.toList)
+
+  /**
+   * Returns a new message that contains all of the given default headers which didn't already
+   * exist (by case-insensitive header name) in this message.
+   */
+  def withDefaultHeaders(defaultHeaders: immutable.Seq[HttpHeader]): Self =
     withHeaders {
-      defaultHeaders.foldLeft(headers) { (acc, h) ⇒ if (acc.exists(_ is h.lowercaseName)) acc else acc :+ h }
+      if (headers.isEmpty) defaultHeaders
+      else defaultHeaders.foldLeft(headers) { (acc, h) ⇒ if (headers.exists(_ is h.lowercaseName)) acc else h +: acc }
     }
 
   /** Returns a copy of this message with the entity set to the given one. */
diff --git a/akka-http-tests/src/test/scala/akka/http/server/directives/RespondWithDirectivesSpec.scala b/akka-http-tests/src/test/scala/akka/http/server/directives/RespondWithDirectivesSpec.scala
index 711a1497a1..d8f0c24e35 100644
--- a/akka-http-tests/src/test/scala/akka/http/server/directives/RespondWithDirectivesSpec.scala
+++ b/akka-http-tests/src/test/scala/akka/http/server/directives/RespondWithDirectivesSpec.scala
@@ -55,7 +55,7 @@ class RespondWithDirectivesSpec extends RoutingSpec {
     "add the given header to a response if the header was missing before" in {
       Get() ~> route() ~> check { response shouldEqual HttpResponse(headers = customHeader :: Nil) }
     }
-    "don't change a response if the header already existed" in {
+    "not change a response if the header already existed" in {
       Get() ~> route(existingHeader) ~> check { response shouldEqual HttpResponse(headers = existingHeader :: Nil) }
     }
   }
@@ -69,9 +69,9 @@ class RespondWithDirectivesSpec extends RoutingSpec {
     "add the given headers to a response if the header was missing before" in {
       Get() ~> route() ~> check { response shouldEqual HttpResponse(headers = customHeader :: customHeader2 :: Nil) }
     }
-    "don't update an existing header" in {
+    "not update an existing header" in {
       Get() ~> route(existingHeader) ~> check {
-        response shouldEqual HttpResponse(headers = existingHeader :: customHeader2 :: Nil)
+        response shouldEqual HttpResponse(headers = List(customHeader2, existingHeader))
       }
     }
   }
diff --git a/akka-http/src/main/scala/akka/http/server/directives/RespondWithDirectives.scala b/akka-http/src/main/scala/akka/http/server/directives/RespondWithDirectives.scala
index fa1b400db1..e0242391b5 100644
--- a/akka-http/src/main/scala/akka/http/server/directives/RespondWithDirectives.scala
+++ b/akka-http/src/main/scala/akka/http/server/directives/RespondWithDirectives.scala
@@ -28,7 +28,7 @@ trait RespondWithDirectives {
    * Unconditionally adds the given response headers to all HTTP responses of its inner Route.
    */
   def respondWithHeaders(responseHeaders: HttpHeader*): Directive0 =
-    respondWithHeaders(responseHeaders.toVector)
+    respondWithHeaders(responseHeaders.toList)
 
   /**
    * Unconditionally adds the given response headers to all HTTP responses of its inner Route.
@@ -41,7 +41,7 @@ trait RespondWithDirectives {
    * if a header already exists it is not added again.
    */
   def respondWithDefaultHeaders(responseHeaders: HttpHeader*): Directive0 =
-    respondWithDefaultHeaders(responseHeaders.toVector)
+    respondWithDefaultHeaders(responseHeaders.toList)
 
   /* Adds the given response headers to all HTTP responses of its inner Route,
    * if a header already exists it is not added again.