typescript props as object or array

type SelectProps = {
  items: SelectItem[]
  selectedItem: SelectItem | SelectItem[]
  setSelectedItem: (arg: SelectItem | SelectItem[]) => void
  multiple?: boolean
}

<Select
            key={idx}
            items={filter.options}
            selectedItem={filter.selectedItems}
            setSelectedItem={filter.setSelectedItems}
          />

Type '(arg: SelectItem[]) => void' is not assignable to type '(arg: SelectItem | SelectItem[]) => void'.
  Types of parameters 'arg' and 'arg' are incompatible.
    Type 'SelectItem | SelectItem[]' is not assignable to type 'SelectItem[]'.
      Type 'SelectItem' is missing the following properties from type 'SelectItem[]': length, pop, push, concat, and 29 more.ts(2322)
Select.tsx(13, 3): The expected type comes from property 'setSelectedItem' which is declared here on type 'IntrinsicAttributes & SelectProps'