Thanks for spotting this, it would be an error. I meant the exact reverse. It’s now updated:

• Passing a nullable string to something that expects a string => clearly an error
• Passing a string to something that expects a nullable string => normally fine

Flow handles this fine for parameters:

acceptsString('foo');
function acceptsString(str: string) {
  acceptsNullableString(str);
}
function acceptsNullableString(str: ?string) {
}

Flow handles this without any problems, and it works fine at runtime, but when it’s the property of an object, things are different (as I try and explain in the article).

Thanks for the correction.