diff --git a/src/controllers/ConversationsController.ts b/src/controllers/ConversationsController.ts
index 5b71d66..983f492 100644
--- a/src/controllers/ConversationsController.ts
+++ b/src/controllers/ConversationsController.ts
@@ -98,8 +98,7 @@ export class ConversationsController {
 	
 		// Update members list
 		if(h.hasPostParameter("members")) {
-
-			const members = h.postNumbersSet("members");
+			const members = await h.postUsersIDSet("members");
 			const canEveryoneAddMembers = await ConversationsHelper.CanEveryoneAddMembers(convID);
 
 			if(!isUserModerator && !canEveryoneAddMembers)
diff --git a/src/entities/BaseRequestsHandler.ts b/src/entities/BaseRequestsHandler.ts
index 1eeb8ce..467294a 100644
--- a/src/entities/BaseRequestsHandler.ts
+++ b/src/entities/BaseRequestsHandler.ts
@@ -179,6 +179,23 @@ export abstract class BaseRequestsHandler implements AbstractUserConnectionConta
 		return new Set(this.postNumbersList(name, minEntries));
 	}
 
+	/**
+	 * Get a list of users included in the POST request
+	 * 
+	 * @param name The name of the POST field
+	 * @param minEntries Minimum number of entries to specify
+	 */
+	public async postUsersIDSet(name: string, minEntries : number = 1) : Promise<Set<number>> {
+		const list = this.postNumbersSet(name, minEntries);
+
+		// Check the users of the list really exist
+		for(const userID of list)
+			if(!await UserHelper.Exists(userID))
+				this.error(404, "User " + userID + " not found!");
+
+		return list;
+	}
+
 	/**
 	 * Attempt to decode JSON included in a POST request
 	 *