diff --git a/src/utils/attendee.js b/src/utils/attendee.js
index 07f577181b..c7bed28409 100644
--- a/src/utils/attendee.js
+++ b/src/utils/attendee.js
@@ -27,6 +27,10 @@
  * @return {string} URI without a mailto prefix
  */
 export function removeMailtoPrefix(uri) {
+	if (typeof uri !== 'string') {
+		return ''
+	}
+
 	if (uri.startsWith('mailto:')) {
 		return uri.substr(7)
 	}
@@ -41,6 +45,10 @@ export function removeMailtoPrefix(uri) {
  * @return {string} URI with a mailto prefix
  */
 export function addMailtoPrefix(uri) {
+	if (typeof uri !== 'string') {
+		return 'mailto:'
+	}
+
 	if (uri.startsWith('mailto:')) {
 		return uri
 	}
diff --git a/tests/javascript/unit/utils/attendee.test.js b/tests/javascript/unit/utils/attendee.test.js
index f6d3412c2f..0b11bc2518 100644
--- a/tests/javascript/unit/utils/attendee.test.js
+++ b/tests/javascript/unit/utils/attendee.test.js
@@ -33,12 +33,22 @@ describe('utils/attendee test suite', () => {
 		expect(removeMailtoPrefix(`mailto:${uri}`)).toEqual(uri)
 	})
 
+	it('should return blank strings when uris are not of type string', () => {
+		expect(removeMailtoPrefix(null)).toEqual('')
+		expect(removeMailtoPrefix(undefined)).toEqual('')
+	})
+
 	it('should add mailto prefixes to uris', () => {
 		const uri = 'principal@test.com'
 		const uriWithPrefix = `mailto:${uri}`
 		expect(addMailtoPrefix(uri)).toEqual(uriWithPrefix)
 		expect(addMailtoPrefix(uriWithPrefix)).toEqual(uriWithPrefix)
 	})
+	
+	it('should add mailto prefixes to uris when they are not of type string', () => {
+		expect(addMailtoPrefix(null)).toEqual("mailto:")
+		expect(addMailtoPrefix(undefined)).toEqual("mailto:")
+	})
 
 	it('should extract a display name of an organizer', () => {
 		const commonName = 'My Name'