From b75ce4fdea6eb10fc474f84a9312f2e0c427cf62 Mon Sep 17 00:00:00 2001
From: Jason Killian <jkillian@palantir.com>
Date: Fri, 15 Jan 2016 15:41:57 -0500
Subject: [PATCH 1/2] Add failing test

---
 .../getPropertySymbolsFromBaseTypesDoesntCrash.ts        | 9 +++++++++
 1 file changed, 9 insertions(+)
 create mode 100644 tests/cases/fourslash/getPropertySymbolsFromBaseTypesDoesntCrash.ts

diff --git a/tests/cases/fourslash/getPropertySymbolsFromBaseTypesDoesntCrash.ts b/tests/cases/fourslash/getPropertySymbolsFromBaseTypesDoesntCrash.ts
new file mode 100644
index 00000000000..0c89de63a43
--- /dev/null
+++ b/tests/cases/fourslash/getPropertySymbolsFromBaseTypesDoesntCrash.ts
@@ -0,0 +1,9 @@
+/// <reference path='fourslash.ts'/>
+
+// @Filename: file1.ts
+//// class ClassA implements IInterface {
+////     private /*1*/value: number;
+//// }
+
+goTo.marker("1");
+verify.documentHighlightsAtPositionCount(1, ["file1.ts"]);
\ No newline at end of file

From 047c62c24025b9ec0086223b9248a464c6d63501 Mon Sep 17 00:00:00 2001
From: Jason Killian <jkillian@palantir.com>
Date: Fri, 15 Jan 2016 15:48:22 -0500
Subject: [PATCH 2/2] Fix issue #6478 (bug in the language services)

---
 src/services/services.ts | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/services/services.ts b/src/services/services.ts
index 8e998760b49..062e3da1bfa 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -6028,6 +6028,10 @@ namespace ts {
              */
             function getPropertySymbolsFromBaseTypes(symbol: Symbol, propertyName: string, result: Symbol[],
                 previousIterationSymbolsCache: SymbolTable): void {
+                if (!symbol) {
+                    return;
+                }
+
                 // If the current symbol is the same as the previous-iteration symbol, we can just return the symbol that has already been visited
                 // This is particularly important for the following cases, so that we do not infinitely visit the same symbol.
                 // For example:
@@ -6043,7 +6047,7 @@ namespace ts {
                     return;
                 }
 
-                if (symbol && symbol.flags & (SymbolFlags.Class | SymbolFlags.Interface)) {
+                if (symbol.flags & (SymbolFlags.Class | SymbolFlags.Interface)) {
                     forEach(symbol.getDeclarations(), declaration => {
                         if (declaration.kind === SyntaxKind.ClassDeclaration) {
                             getPropertySymbolFromTypeReference(getClassExtendsHeritageClauseElement(<ClassDeclaration>declaration));