"Method expression is not of function type" error on Typescript constructor call

Using WebStorm 7.0.3.

The IDE is complaining of a "Method expression is not of function type" error on a constructor call, yet the typescript compiler compiles it with no errors, and it runs with no errors. I cannot seem to make the error go away.

The most minimal reproduction I've beel able to create has the following files:

//a.ts
class A {
     constructor() {}
     public getX():number {
          return 3;
     }
}
export = A;

//package.json for a
{
     "name": "a",
     "main": "./a.js"
}

//main.ts
/// <reference path="tsd/node.d.ts" />
var A = require("a");
var a = new A(); // Error is here, on the A() constructor call.
console.log(a.getX()); // When compiled and run, still outputs '3' as expected.

Directory structure looks like this:

test
|-- main.ts
|-- node_modules
|   `-- a
|       |-- a.ts
|       `-- package.json
`-- tsd
    `-- node.d.ts

I get the feeling I'm probably doing something stupid, but just cannot seem to fix it, so any suggestions are welcome.