diff --git a/components/app.js b/components/app.js
index e3d35ff..668a7a5 100644
--- a/components/app.js
+++ b/components/app.js
@@ -16,7 +16,7 @@ import ScrollManager from "./scroll-manager.js";
 import Dialog from "./dialog.js";
 import { html, Component, createRef } from "../lib/index.js";
 import { strip as stripANSI } from "../lib/ansi.js";
-import { SERVER_BUFFER, BufferType, ReceiptType, ServerStatus, Unread, State, getServerName } from "../state.js";
+import { SERVER_BUFFER, BufferType, ReceiptType, ServerStatus, Unread, State, getServerName, isMessageBeforeReceipt } from "../state.js";
 import commands from "../commands.js";
 import { setup as setupKeybindings } from "../keybindings.js";
 import * as store from "../store.js";
@@ -463,7 +463,7 @@ export default class App extends Component {
 
 	hasReceipt(target, type, msg) {
 		let receipt = this.getReceipt(target, type);
-		return receipt && msg.tags.time <= receipt.time;
+		return isMessageBeforeReceipt(msg, receipt);
 	}
 
 	setReceipt(target, type, msg) {
@@ -596,7 +596,7 @@ export default class App extends Component {
 			}
 
 			// Don't show unread marker for my own messages
-			if (client.isMyNick(msg.prefix.name) && (!prevReadReceipt || prevReadReceipt.time < msg.tags.time)) {
+			if (client.isMyNick(msg.prefix.name) && !isMessageBeforeReceipt(msg, prevReadReceipt)) {
 				prevReadReceipt = receiptFromMessage(msg);
 			}
 
diff --git a/components/buffer.js b/components/buffer.js
index eaae0e1..0db06b3 100644
--- a/components/buffer.js
+++ b/components/buffer.js
@@ -2,7 +2,7 @@ import { html, Component } from "../lib/index.js";
 import linkify from "../lib/linkify.js";
 import * as irc from "../lib/irc.js";
 import { strip as stripANSI } from "../lib/ansi.js";
-import { BufferType, ServerStatus, getNickURL, getChannelURL, getMessageURL } from "../state.js";
+import { BufferType, ServerStatus, getNickURL, getChannelURL, getMessageURL, isMessageBeforeReceipt } from "../state.js";
 import * as store from "../store.js";
 import Membership from "./membership.js";
 
@@ -633,7 +633,7 @@ export default class Buffer extends Component {
 		buf.messages.forEach((msg) => {
 			let sep = [];
 
-			if (!hasUnreadSeparator && buf.type != BufferType.SERVER && buf.prevReadReceipt && msg.tags.time > buf.prevReadReceipt.time) {
+			if (!hasUnreadSeparator && buf.type != BufferType.SERVER && !isMessageBeforeReceipt(msg, buf.prevReadReceipt)) {
 				sep.push(html`<${UnreadSeparator} key="unread"/>`);
 				hasUnreadSeparator = true;
 			}
diff --git a/state.js b/state.js
index f1d5a7b..a2c04d5 100644
--- a/state.js
+++ b/state.js
@@ -85,6 +85,19 @@ export function getServerName(server, bouncerNetwork) {
 	}
 }
 
+export function isMessageBeforeReceipt(msg, receipt) {
+	if (!receipt) {
+		return false;
+	}
+	if (!msg.tags.time) {
+		throw new Error("Missing time message tag");
+	}
+	if (!receipt.time) {
+		throw new Error("Missing receipt time");
+	}
+	return msg.tags.time <= receipt.time;
+}
+
 function updateState(state, updater) {
 	let updated;
 	if (typeof updater === "function") {