How do I convert missing values into strings?

I have a Deedle DataFrame of type Frame<int,string> that contains some missing values. I would like to convert the missing values into empty strings "". I tried to use the valueOr function but that did not help. Is there a way to do this?

Here is my DataFrame:

let s1 = Series.ofOptionalObservations [ 1 => Some("A"); 2 => None ]
let s2 = Series.ofOptionalObservations [ 1 => Some("B"); 2 => Some("C") ]
let df = Frame.ofColumns ["A", s1; "BC", s2]

Typing df;; in FSI yields some information including

ColumnTypes = seq [System.String; System.String];. So the values of df are of type string and not string option.

This is the function valueOr:

let valueOr (someDefault: 'a) (xo: 'a option) : 'a =
    match xo with
    | Some v -> v
    | None -> someDefault

I defined an auxiliary function emptyFoo as:

let emptyFoo = valueOr ""

The signature of emptyFoo is string option -> string. This means emptyFoo should not be acceptable to the compiler in the following command:

let df' = Frame.mapValues emptyFoo df

This is because the values of df are of type string and not string option.

Still, the compiler does not complain and the code runs. However, df' still has a missing value.

Is there a way to transform the missing value into the empty string?