diff --git a/downstream.go b/downstream.go
index ef61d34..8d6a9ff 100644
--- a/downstream.go
+++ b/downstream.go
@@ -555,7 +555,9 @@ func (dc *downstreamConn) register() error {
 
 		var seqPtr *uint64
 		if firstDownstream {
+			uc.lock.Lock()
 			seq, ok := uc.history[historyName]
+			uc.lock.Unlock()
 			if ok {
 				seqPtr = &seq
 			}
@@ -583,7 +585,9 @@ func (dc *downstreamConn) register() error {
 			dc.user.lock.Unlock()
 
 			if lastDownstream {
+				uc.lock.Lock()
 				uc.history[historyName] = seq
+				uc.lock.Unlock()
 			}
 		}()
 	})
diff --git a/upstream.go b/upstream.go
index 7fc2a32..a84f17f 100644
--- a/upstream.go
+++ b/upstream.go
@@ -8,6 +8,7 @@ import (
 	"net"
 	"strconv"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/emersion/go-sasl"
@@ -48,11 +49,13 @@ type upstreamConn struct {
 	closed     bool
 	modes      modeSet
 	channels   map[string]*upstreamChannel
-	history    map[string]uint64
 	caps       map[string]string
 
 	saslClient  sasl.Client
 	saslStarted bool
+
+	lock    sync.Mutex
+	history map[string]uint64 // TODO: move to network
 }
 
 func connectToUpstream(network *network) (*upstreamConn, error) {