No Build Command Defined error in Visual Studio Code

When I press CMD+SHIFT+B in Visual Studio Code (to compile typescript to javascript) I get the following error:

No build task defined. Mark a task with 'isBuildCommand' in the tasks.json file.

My tasks.json file looks like this. This has always worked in any other Typescript project.

{
    "version": "0.1.0",
    "command": "tsc",
    "isShellCommand": true,
    "args": ["-w"],
    "problemMatcher": "$tsc"
}

It doesn't work, and even when I add 'isBuildCommand', I still get the same error!

{
    "version": "0.1.0",
    "command": "tsc",
    "isBuildCommand": true,
    "args": ["-w"],
    "problemMatcher": "$tsc"
}