diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 40a3de7553f..ef2c589c393 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -96,7 +96,6 @@ namespace ts {
             getTypeAtLocation: getTypeOfNode,
             getPropertySymbolOfDestructuringAssignment,
             signatureToString,
-            indexSignatureToString,
             typeToString,
             getSymbolDisplayBuilder,
             symbolToString,
@@ -2037,15 +2036,6 @@ namespace ts {
             return result;
         }
 
-        function indexSignatureToString(info: IndexInfo, kind: IndexKind, enclosingDeclaration?: Node): string {
-            const writer = getSingleLineStringWriter();
-            getSymbolDisplayBuilder().buildIndexSignatureDisplay(info, writer, kind, enclosingDeclaration);
-            const result = writer.string();
-            releaseStringWriter(writer);
-
-            return result;
-        }
-
         function typeToString(type: Type, enclosingDeclaration?: Node, flags?: TypeFormatFlags): string {
             const writer = getSingleLineStringWriter();
             getSymbolDisplayBuilder().buildTypeDisplay(type, writer, enclosingDeclaration, flags);
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index d93aea8d26a..2a04916d45d 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2353,7 +2353,6 @@ namespace ts {
         getTypeAtLocation(node: Node): Type;
         getTypeFromTypeReference(node: TypeReferenceNode | ExpressionWithTypeArguments | JSDocTypeReference): Type;
         signatureToString(signature: Signature, enclosingDeclaration?: Node, flags?: TypeFormatFlags, kind?: SignatureKind): string;
-        indexSignatureToString(info: IndexInfo, kind: IndexKind, enclosingDeclaration?: Node): string;
         typeToString(type: Type, enclosingDeclaration?: Node, flags?: TypeFormatFlags): string;
         symbolToString(symbol: Symbol, enclosingDeclaration?: Node, meaning?: SymbolFlags): string;
         createSymbol(flags: SymbolFlags, name: string): Symbol;
diff --git a/src/services/codefixes/fixClassIncorrectlyImplementsInterface.ts b/src/services/codefixes/fixClassIncorrectlyImplementsInterface.ts
index 7afbbbfd6a1..915b23f050b 100644
--- a/src/services/codefixes/fixClassIncorrectlyImplementsInterface.ts
+++ b/src/services/codefixes/fixClassIncorrectlyImplementsInterface.ts
@@ -29,7 +29,7 @@ namespace ts.codefix {
             // Note that this is ultimately derived from a map indexed by symbol names,
             // so duplicates cannot occur.
             const implementedTypeSymbols = checker.getPropertiesOfType(implementedType);
-            const nonPrivateMembers = implementedTypeSymbols.filter(symbolRefersToNonPrivateMember);
+            const nonPrivateMembers = implementedTypeSymbols.filter(symbol => !(getModifierFlags(symbol.valueDeclaration) & ModifierFlags.Private));
 
             let insertion = getMissingIndexSignatureInsertion(implementedType, IndexKind.Number, classDecl, hasNumericIndexSignature);
             insertion += getMissingIndexSignatureInsertion(implementedType, IndexKind.String, classDecl, hasStringIndexSignature);
@@ -47,18 +47,17 @@ namespace ts.codefix {
             if (!hasIndexSigOfKind) {
                 const IndexInfoOfKind = checker.getIndexInfoOfType(type, kind);
                 if (IndexInfoOfKind) {
-                    return checker.indexSignatureToString(IndexInfoOfKind, kind, enclosingDeclaration);
+                    const writer = getSingleLineStringWriter();
+                    checker.getSymbolDisplayBuilder().buildIndexSignatureDisplay(IndexInfoOfKind, writer, kind, enclosingDeclaration);
+                    const result = writer.string();
+                    releaseStringWriter(writer);
+
+                    return result;
                 }
             }
             return "";
         }
 
-        function symbolRefersToNonPrivateMember(symbol: Symbol): boolean {
-            const decls = symbol.getDeclarations();
-            Debug.assert(!!(decls && decls.length > 0));
-            return !(getModifierFlags(decls[0]) & ModifierFlags.Private);
-        }
-
         function pushAction(result: CodeAction[], insertion: string, description: string): void {
             const newAction: CodeAction = {
                 description: description,