diff --git a/src/github.com/matrix-org/dendrite/clientapi/routing/register.go b/src/github.com/matrix-org/dendrite/clientapi/routing/register.go
index 09c8c4e1d..8d3ca2247 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/routing/register.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/routing/register.go
@@ -50,26 +50,36 @@ const (
 	sessionIDLength   = 24
 )
 
+// sessionsDict represents every sessions' completed flow stages.
 type sessionsDict struct {
 	sessions map[string][]authtypes.LoginType
 }
 
-func (d sessionsDict) Get(key string) []authtypes.LoginType {
-	if v, ok := d.sessions[key]; ok {
-		return v
+// GetCompletedStages returns the completed stages for a session.
+func (d sessionsDict) GetCompletedStages(sessionID string) []authtypes.LoginType {
+	if completedStages, ok := d.sessions[sessionID]; ok {
+		return completedStages
 	}
+	// Ensure that a empty slice is return and not nil. See gh #399.
 	return make([]authtypes.LoginType, 0)
 }
 
-func (d *sessionsDict) AddCompletedStage(key string, v authtypes.LoginType) {
-	d.sessions[key] = append(d.Get(key), v)
+// AddCompletedStage adds a completed stage to the session.
+func (d *sessionsDict) AddCompletedStage(sessionID string, stage authtypes.LoginType) {
+	d.sessions[sessionID] = append(d.GetCompletedStages(sessionID), stage)
+}
+
+// newSessionsDict returns a sessionsDict whose contained map is initialized and empty.
+func newSessionsDict() *sessionsDict {
+	return &sessionsDict{
+		sessions: make(map[string][]authtypes.LoginType),
+	}
 }
 
 var (
 	// TODO: Remove old sessions. Need to do so on a session-specific timeout.
-	sessions = sessionsDict{ // Sessions and completed flow stages
-		sessions: make(map[string][]authtypes.LoginType),
-	}
+	// sessions stores the completed flow stages for all sessions. Referenced using their sessionID.
+	sessions           = newSessionsDict()
 	validUsernameRegex = regexp.MustCompile(`^[0-9a-z_\-./]+$`)
 )
 
@@ -129,7 +139,7 @@ func newUserInteractiveResponse(
 	params map[string]interface{},
 ) userInteractiveResponse {
 	return userInteractiveResponse{
-		fs, sessions.Get(sessionID), params, sessionID,
+		fs, sessions.GetCompletedStages(sessionID), params, sessionID,
 	}
 }
 
@@ -495,7 +505,8 @@ func handleRegistrationFlow(
 	// Check if the user's registration flow has been completed successfully
 	// A response with current registration flow and remaining available methods
 	// will be returned if a flow has not been successfully completed yet
-	return checkAndCompleteFlow(sessions.Get(sessionID), req, r, sessionID, cfg, accountDB, deviceDB)
+	return checkAndCompleteFlow(sessions.GetCompletedStages(sessionID),
+		req, r, sessionID, cfg, accountDB, deviceDB)
 }
 
 // checkAndCompleteFlow checks if a given registration flow is completed given