diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 1891eba1591..5b95f90e115 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -9125,7 +9125,7 @@ namespace ts {
             return type;
         }
 
-        function getTypeArguments(type: TypeReference): ReadonlyArray<Type> {
+        function getTypeArguments(type: TypeReference): readonly Type[] {
             if (!type.resolvedTypeArguments) {
                 const node = type.node;
                 const typeArguments = !node ? emptyArray :
diff --git a/src/compiler/symbolWalker.ts b/src/compiler/symbolWalker.ts
index 76bffca9740..b03ef31db6e 100644
--- a/src/compiler/symbolWalker.ts
+++ b/src/compiler/symbolWalker.ts
@@ -11,7 +11,7 @@ namespace ts {
         getIndexTypeOfStructuredType: (type: Type, kind: IndexKind) => Type | undefined,
         getConstraintOfTypeParameter: (typeParameter: TypeParameter) => Type | undefined,
         getFirstIdentifier: (node: EntityNameOrEntityNameExpression) => Identifier,
-        getTypeArguments: (type: TypeReference) => ReadonlyArray<Type>) {
+        getTypeArguments: (type: TypeReference) => readonly Type[]) {
 
         return getSymbolWalker;