How can i solve this problem with typescript?

I have an interface in typescript to describe the data type of a structure. The problem is that when accessing this structure within an if, it gives me a typescript error that I don't understand and can't resolve. The code works.

This is the interface

interface ISignUpFormData {
  username: {
    value: string;
    valid: boolean;
    unique: boolean;
  };
  email: { value: string; valid: boolean; unique: boolean };
  password: { value: string; valid: boolean };
  birthdate: { value: string; valid: boolean };
}

And so I access the data

for (let data in formData) {
      if (
        formData[data].value === "" ||
        !formData[data].valid ||
        !formData[data].unique
      ) {
        canContinue = false;
        errors.push(data);
      }
    }

In the if condition lines I get this error:

The element implicitly has type "any" because the expression of type "string" cannot be used to index the type "ISignUpFormData".
  No index signature with a parameter of type "string" was found on type "ISignUpFormData".