diff --git a/src/relative-time-element.ts b/src/relative-time-element.ts
index c943a2c..3c48216 100644
--- a/src/relative-time-element.ts
+++ b/src/relative-time-element.ts
@@ -4,10 +4,10 @@ import {RelativeTimeFormat as RelativeTimeFormatPonyfill} from './relative-time-
 import {isDuration, withinDuration} from './duration.js'
 import {strftime} from './strftime.js'
 
-const supportsIntlDatetime = 'Intl' in window && 'DateTimeFormat' in Intl
+const supportsIntlDatetime = typeof Intl !== 'undefined' && 'DateTimeFormat' in Intl
 const DateTimeFormat = supportsIntlDatetime ? Intl.DateTimeFormat : DateTimeFormatPonyFill
 
-const supportsIntlRelativeTime = 'Intl' in window && 'RelativeTimeFormat' in Intl
+const supportsIntlRelativeTime = typeof Intl !== 'undefined' && 'RelativeTimeFormat' in Intl
 const RelativeTimeFormat = supportsIntlRelativeTime ? Intl.RelativeTimeFormat : RelativeTimeFormatPonyfill
 
 export type Format = 'auto' | 'micro' | 'elapsed' | string
diff --git a/src/strftime.ts b/src/strftime.ts
index f5b6c14..0155586 100644
--- a/src/strftime.ts
+++ b/src/strftime.ts
@@ -1,4 +1,4 @@
-const supportsIntlDatetime = 'Intl' in window && 'DateTimeFormat' in Intl
+const supportsIntlDatetime = typeof Intl !== 'undefined' && 'DateTimeFormat' in Intl
 
 const weekdays = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday']
 const months = [