diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index e2c6e4d6a53..f75ba15254b 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -79,6 +79,7 @@ namespace ts {
             getDeclaredTypeOfSymbol,
             getPropertiesOfType,
             getPropertyOfType,
+            getIndexInfoOfType,
             getSignaturesOfType,
             getIndexTypeOfType,
             getBaseTypes,
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index eb9584f638a..81b4005ae5e 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2335,6 +2335,7 @@ namespace ts {
         getDeclaredTypeOfSymbol(symbol: Symbol): Type;
         getPropertiesOfType(type: Type): Symbol[];
         getPropertyOfType(type: Type, propertyName: string): Symbol;
+        getIndexInfoOfType(type: Type, kind: IndexKind): IndexInfo;
         getSignaturesOfType(type: Type, kind: SignatureKind): Signature[];
         getIndexTypeOfType(type: Type, kind: IndexKind): Type;
         getBaseTypes(type: InterfaceType): ObjectType[];
diff --git a/src/services/codefixes/fixClassIncorrectlyImplementsInterface.ts b/src/services/codefixes/fixClassIncorrectlyImplementsInterface.ts
index e82c579a54f..11db00eb43c 100644
--- a/src/services/codefixes/fixClassIncorrectlyImplementsInterface.ts
+++ b/src/services/codefixes/fixClassIncorrectlyImplementsInterface.ts
@@ -31,14 +31,14 @@ namespace ts.codefix {
                 let insertion = "";
 
                 if (!hasNumericIndexSignature) {
-                    const typeNumericIndexInfo = implementedType.declaredNumberIndexInfo;
+                    const typeNumericIndexInfo = checker.getIndexInfoOfType(implementedType, IndexKind.Number);
                     if (typeNumericIndexInfo) {
                         insertion = checker.indexSignatureToString(typeNumericIndexInfo, SyntaxKind.NumberKeyword, classDecl);
                     }
                 }
 
                 if (!hasStringIndexSignature) {
-                    const typeStringIndexInfo = implementedType.declaredStringIndexInfo;
+                    const typeStringIndexInfo = checker.getIndexInfoOfType(implementedType, IndexKind.String);
                     if (typeStringIndexInfo) {
                         insertion += checker.indexSignatureToString(typeStringIndexInfo, SyntaxKind.StringKeyword, classDecl);
                     }
diff --git a/tests/cases/fourslash/codeFixUnImplementedInterfaceIndexSignaturesString.ts b/tests/cases/fourslash/codeFixUnImplementedInterfaceIndexSignaturesString.ts
index 37abd751382..4b595c6eda7 100644
--- a/tests/cases/fourslash/codeFixUnImplementedInterfaceIndexSignaturesString.ts
+++ b/tests/cases/fourslash/codeFixUnImplementedInterfaceIndexSignaturesString.ts
@@ -1,10 +1,10 @@
 /// <reference path='fourslash.ts' />
 
-//// interface I {
-////     [x: string]: number;
+//// interface I<X> {
+////     [x: string]: X;
 //// }
 ////
-//// class C implements I {[| |]}
+//// class C implements I<number> {[| |]}
 
 verify.rangeAfterCodeFix(`
     [x: string]: number;