Declaring a custom function in TypeScript with error support

Question

I'm developing an Excel add-in with a suite of custom functions. Many take arrays as input, and I would like said functions to return valid results for valid inputs and #VALUE for invalid inputs, on a per-cell basis. My question is, how do I declare the function in TypeScript?

This compiles:

export function doSomething(values: number[][]): any[][]
  return values.map(rowValues =>
    rowValues.map(value =>
      value >= 0 ? String(value) : new CustomFunctions.Error(CustomFunctions.ErrorCode.invalidValue, "No negatives!")
    )
  );
}

This doesn't:

export function doSomething(values: number[][]): (string | CustomFunctions.Error)[][]
  return values.map(rowValues =>
    rowValues.map(value =>
      value >= 0 ? String(value) : new CustomFunctions.Error(CustomFunctions.ErrorCode.invalidValue, "No negatives!")
    )
  );
}

The latter is better TypeScript, in that it enforces type safety on the results, but I get the following error in the task pane:

functions.ts Type doesn't match mappings (90,116)

How do I declare the return type for functions that can return errors in an array?