Having a hard time debugging error – Token ‘{‘ invalid key at column 2

I notice that this error also happens when binding data to an attribute on a custom directive. Where

$scope.myData.value = "Hello!";

This causes the error:

<my-custom-directive my-attr="{{myData.value}}"></my-custom-directive>

But this works fine:

<my-custom-directive my-attr="myData.value"></my-custom-directive>