diff --git a/lib/shared_types.ts b/lib/shared_types.ts
index 8902820eb..818b45bfe 100644
--- a/lib/shared_types.ts
+++ b/lib/shared_types.ts
@@ -76,9 +76,17 @@ export interface UserProfile {
   experiment_bucket_map: ExperimentBucketMap;
 }
 
+export type EventPropertyValue = string | number | boolean;
+
+export type EventProperties = {
+  '$opt_event_properties'?: {
+    [key: string]: EventPropertyValue;
+  };
+};
+
 export type EventTags = {
   [key: string]: string | number | null;
-};
+} | EventProperties;
 
 export interface UserProfileService {
   lookup(userId: string): UserProfile;