diff --git a/src/main/scala/sangria/marshalling/ArrayMapBuilder.scala b/src/main/scala/sangria/marshalling/ArrayMapBuilder.scala
index 5290b0e..f822715 100644
--- a/src/main/scala/sangria/marshalling/ArrayMapBuilder.scala
+++ b/src/main/scala/sangria/marshalling/ArrayMapBuilder.scala
@@ -12,7 +12,7 @@ class ArrayMapBuilder[T](keys: Seq[String]) extends Iterable[(String, T)] {
   private val indexLookup = keys.iterator.zipWithIndex.toMap
   private val indexesSet = MutableSet[Int]()
 
-  def add(key: String, elem: T) = {
+  def add(key: String, elem: T): ArrayMapBuilder[T] = {
     val idx = indexLookup(key)
 
     elements(idx) = key -> elem
@@ -65,9 +65,9 @@ class ArrayMapBuilder[T](keys: Seq[String]) extends Iterable[(String, T)] {
 
   override def iterator: Iterator[(String, T)] =
     new Iterator[(String, T)] {
-      var index = -1
-      var nextIndex = -1
-      val elementsLength = elements.length
+      private var index = -1
+      private var nextIndex = -1
+      private val elementsLength = elements.length
 
       @tailrec def nextIndex(current: Int): Int = {
         val next = current + 1
diff --git a/src/main/scala/sangria/marshalling/FromInput.scala b/src/main/scala/sangria/marshalling/FromInput.scala
index 7d91ad2..a5367f3 100644
--- a/src/main/scala/sangria/marshalling/FromInput.scala
+++ b/src/main/scala/sangria/marshalling/FromInput.scala
@@ -1,7 +1,6 @@
 package sangria.marshalling
 
 import scala.annotation.implicitNotFound
-import scala.language.higherKinds
 
 @implicitNotFound(
   "Type ${Val} cannot be used as an input. Please consider defining an implicit instance of `FromInput` for it.")
diff --git a/src/main/scala/sangria/marshalling/MarshallingUtil.scala b/src/main/scala/sangria/marshalling/MarshallingUtil.scala
index 0abf2fb..01a7700 100644
--- a/src/main/scala/sangria/marshalling/MarshallingUtil.scala
+++ b/src/main/scala/sangria/marshalling/MarshallingUtil.scala
@@ -35,7 +35,7 @@ object MarshallingUtil {
   }
 
   implicit class MarshaledConverter[In: InputUnmarshaller](in: In) {
-    def convertMarshaled[Out: ResultMarshallerForType] = convert(in)
+    def convertMarshaled[Out: ResultMarshallerForType]: Out = convert(in)
   }
 
   implicit class ResultMarshallerOps(val m: ResultMarshaller) extends AnyVal {
diff --git a/src/main/scala/sangria/marshalling/ToInput.scala b/src/main/scala/sangria/marshalling/ToInput.scala
index 669aea2..23c10ee 100644
--- a/src/main/scala/sangria/marshalling/ToInput.scala
+++ b/src/main/scala/sangria/marshalling/ToInput.scala
@@ -3,7 +3,6 @@ package sangria.marshalling
 import sangria.util.tag._
 
 import scala.annotation.implicitNotFound
-import scala.language.higherKinds
 
 @implicitNotFound(
   "Type ${Val} cannot be used as a default value. Please consider defining an implicit instance of `ToInput` for it.")