diff --git a/src/components/layout/Hairline/index.js b/src/components/layout/Hairline/index.js
index dae5ea6f..0dd0acb3 100644
--- a/src/components/layout/Hairline/index.js
+++ b/src/components/layout/Hairline/index.js
@@ -3,7 +3,7 @@ import * as React from 'react'
 import { type Size, getSize } from '~/theme/size'
 import { border } from '~/theme/variables'
 
-const calculateStyleFrom = (margin: Size) => ({
+const calculateStyleFrom = (margin?: Size) => ({
   width: '100%',
   height: '1px',
   backgroundColor: border,
diff --git a/src/theme/size.js b/src/theme/size.js
index 1d0f5526..49098ac1 100644
--- a/src/theme/size.js
+++ b/src/theme/size.js
@@ -3,7 +3,7 @@ import { xs, sm, md, lg, xl } from '~/theme/variables'
 
 export type Size = 'xs' | 'sm' | 'md' | 'lg' | 'xl'
 
-export const getSize = (size: Size) => {
+export const getSize = (size: Size | typeof undefined) => {
   switch (size) {
   case 'xs':
     return xs