Confusing Behavior with Type Error in TypeScript Schema

Schema.array(Schema.null).pipe(Schema.maxLength(1))

<repl>.ts:17:32 - error TS2345: Argument of type '<I>(self: Schema<I, string>) => Schema<I, string>' is not assignable to parameter of type '(_: Schema<readonly null[], readonly null[]>) => Schema<readonly null[], string
>'.
  Types of parameters 'self' and '_' are incompatible.
    Type 'Schema<readonly null[], readonly null[]>' is not assignable to type 'Schema<readonly null[], string>'.
      The types of '[TypeId].To' are incompatible between these types.
        Type '(_: readonly null[]) => readonly null[]' is not assignable to type '(_: string) => string'.
          Types of parameters '_' and '_' are incompatible.
            Type 'string' is not assignable to type 'readonly null[]'.

17 Schema.array(Schema.null).pipe(Schema.maxLength(1))
                                  ~~~~~~~~~~~~~~~~~~~

Schema.struct({
  field: Schema.array(Schema.null).pipe(Schema.maxLength(1))
})