Address PR

Author: Yui T

src/compiler/binder.ts

@@ -2178,11 +2178,10 @@ namespace ts {
                 case SyntaxKind.JSDocRecordMember:
                     return bindPropertyWorker(node as JSDocRecordMember);
                 case SyntaxKind.JSDocPropertyTag:
-                    let optionalType = 0;
-                    if ((node as JSDocPropertyTag).typeExpression.type.kind === SyntaxKind.JSDocOptionalType) {
-                        optionalType = SymbolFlags.Optional;
-                    }
-                    return declareSymbolAndAddToSymbolTable(node as JSDocPropertyTag, SymbolFlags.Property | optionalType, SymbolFlags.PropertyExcludes);
+                    return declareSymbolAndAddToSymbolTable(node as JSDocPropertyTag,
+                        (node as JSDocPropertyTag).typeExpression && (node as JSDocPropertyTag).typeExpression.type.kind === SyntaxKind.JSDocOptionalType ?
+                            SymbolFlags.Property | SymbolFlags.Optional : SymbolFlags.Property,
+                        SymbolFlags.PropertyExcludes);
                 case SyntaxKind.JSDocFunctionType:
                     return bindFunctionOrConstructorType(<SignatureDeclaration>node);
                 case SyntaxKind.JSDocTypeLiteral: