diff --git a/clientapi/routing/register.go b/clientapi/routing/register.go
index 0af407587..d0f36a6fd 100644
--- a/clientapi/routing/register.go
+++ b/clientapi/routing/register.go
@@ -29,6 +29,7 @@ import (
 	"sort"
 	"strconv"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/matrix-org/dendrite/common/config"
@@ -70,12 +71,17 @@ func init() {
 }
 
 // sessionsDict keeps track of completed auth stages for each session.
+// It shouldn't be passed by value because it contains a mutex.
 type sessionsDict struct {
+	sync.Mutex
 	sessions map[string][]authtypes.LoginType
 }
 
 // GetCompletedStages returns the completed stages for a session.
-func (d sessionsDict) GetCompletedStages(sessionID string) []authtypes.LoginType {
+func (d *sessionsDict) GetCompletedStages(sessionID string) []authtypes.LoginType {
+	d.Lock()
+	defer d.Unlock()
+
 	if completedStages, ok := d.sessions[sessionID]; ok {
 		return completedStages
 	}
@@ -91,12 +97,15 @@ func newSessionsDict() *sessionsDict {
 
 // AddCompletedSessionStage records that a session has completed an auth stage.
 func AddCompletedSessionStage(sessionID string, stage authtypes.LoginType) {
-	for _, completedStage := range sessions.GetCompletedStages(sessionID) {
+	sessions.Lock()
+	defer sessions.Unlock()
+
+	for _, completedStage := range sessions.sessions[sessionID] {
 		if completedStage == stage {
 			return
 		}
 	}
-	sessions.sessions[sessionID] = append(sessions.GetCompletedStages(sessionID), stage)
+	sessions.sessions[sessionID] = append(sessions.sessions[sessionID], stage)
 }
 
 var (