diff --git a/gencapdefs.py b/gencapdefs.py
index f3825c51..23efda52 100644
--- a/gencapdefs.py
+++ b/gencapdefs.py
@@ -113,7 +113,7 @@ CAPDEFS = [
     ),
     CapDef(
         identifier="Resume",
-        name="draft/resume-0.4",
+        name="draft/resume-0.5",
         url="https://github.com/DanielOaks/ircv3-specifications/blob/master+resume/extensions/resume.md",
         standard="proposed IRCv3",
     ),
diff --git a/irc/caps/defs.go b/irc/caps/defs.go
index 0ac48337..6a691bbc 100644
--- a/irc/caps/defs.go
+++ b/irc/caps/defs.go
@@ -77,7 +77,7 @@ const (
 	// https://github.com/SaberUK/ircv3-specifications/blob/rename/extensions/rename.md
 	Rename Capability = iota
 
-	// Resume is the proposed IRCv3 capability named "draft/resume-0.4":
+	// Resume is the proposed IRCv3 capability named "draft/resume-0.5":
 	// https://github.com/DanielOaks/ircv3-specifications/blob/master+resume/extensions/resume.md
 	Resume Capability = iota
 
@@ -141,7 +141,7 @@ var (
 		"message-tags",
 		"multi-prefix",
 		"draft/rename",
-		"draft/resume-0.4",
+		"draft/resume-0.5",
 		"sasl",
 		"server-time",
 		"draft/setname",
diff --git a/irc/client.go b/irc/client.go
index add5b3d4..f28153db 100644
--- a/irc/client.go
+++ b/irc/client.go
@@ -579,9 +579,6 @@ func (session *Session) playResume() {
 	}
 
 	timestamp := session.resumeDetails.Timestamp
-	if timestamp.IsZero() {
-		timestamp = session.client.ctime
-	}
 	gap := lastDiscarded.Sub(timestamp)
 	session.resumeDetails.HistoryIncomplete = gap > 0
 	gapSeconds := int(gap.Seconds()) + 1 // round up to avoid confusion
diff --git a/irc/commands.go b/irc/commands.go
index f5049948..2b221621 100644
--- a/irc/commands.go
+++ b/irc/commands.go
@@ -232,7 +232,7 @@ func init() {
 		"RESUME": {
 			handler:      resumeHandler,
 			usablePreReg: true,
-			minParams:    1,
+			minParams:    2,
 		},
 		"SAJOIN": {
 			handler:   sajoinHandler,
diff --git a/irc/handlers.go b/irc/handlers.go
index e6cf61ce..98b77c8d 100644
--- a/irc/handlers.go
+++ b/irc/handlers.go
@@ -2359,27 +2359,24 @@ func renameHandler(server *Server, client *Client, msg ircmsg.IrcMessage, rb *Re
 	return false
 }
 
-// RESUME <token> [timestamp]
+// RESUME <token> <timestamp>
 func resumeHandler(server *Server, client *Client, msg ircmsg.IrcMessage, rb *ResponseBuffer) bool {
-	details := ResumeDetails{
-		PresentedToken: msg.Params[0],
-	}
-
 	if client.registered {
 		rb.Add(nil, server.name, "FAIL", "RESUME", "REGISTRATION_IS_COMPLETED", client.t("Cannot resume connection, connection registration has already been completed"))
 		return false
 	}
 
-	if 1 < len(msg.Params) {
-		ts, err := time.Parse(IRCv3TimestampFormat, msg.Params[1])
-		if err == nil {
-			details.Timestamp = ts
-		} else {
-			rb.Add(nil, server.name, "WARN", "RESUME", "HISTORY_LOST", client.t("Timestamp is not in 2006-01-02T15:04:05.999Z format, ignoring it"))
-		}
+	ts, err := time.Parse(IRCv3TimestampFormat, msg.Params[1])
+	if err != nil {
+		rb.Add(nil, server.name, "FAIL", "RESUME", "INVALID_TIMESTAMP", client.t("Cannot resume connection, timestamp is not valid"))
+		return false
+	}
+
+	rb.session.resumeDetails = &ResumeDetails{
+		PresentedToken: msg.Params[0],
+		Timestamp:      ts,
 	}
 
-	rb.session.resumeDetails = &details
 	return false
 }