diff --git a/clientapi/routing/register.go b/clientapi/routing/register.go
index af2e99ed0..7d84f2494 100644
--- a/clientapi/routing/register.go
+++ b/clientapi/routing/register.go
@@ -31,6 +31,7 @@ import (
 
 	"github.com/matrix-org/dendrite/internal/eventutil"
 	"github.com/matrix-org/dendrite/setup/config"
+	"github.com/tidwall/gjson"
 
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/gomatrixserverlib/tokens"
@@ -525,22 +526,37 @@ func Register(
 	userAPI userapi.UserRegisterAPI,
 	cfg *config.ClientAPI,
 ) util.JSONResponse {
+	defer req.Body.Close() // nolint: errcheck
+	reqBody, err := ioutil.ReadAll(req.Body)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.NotJSON("Unable to read request body"),
+		}
+	}
+
 	var r registerRequest
-	resErr := httputil.UnmarshalJSONRequest(req, &r)
-	if resErr != nil {
+	sessionID := gjson.GetBytes(reqBody, "auth.session").String()
+	if sessionID == "" {
+		// Generate a new, random session ID
+		sessionID = util.RandomString(sessionIDLength)
+	} else if data, ok := sessions.getParams(sessionID); ok {
+		// Use the parameters from the session as our defaults.
+		// Some of these might end up being overwritten if the
+		// values are specified again in the request body.
+		r.Username = data.Username
+		r.Password = data.Password
+		r.DeviceID = data.DeviceID
+		r.InitialDisplayName = data.InitialDisplayName
+		r.InhibitLogin = data.InhibitLogin
+	}
+	if resErr := httputil.UnmarshalJSON(reqBody, &r); resErr != nil {
 		return *resErr
 	}
 	if req.URL.Query().Get("kind") == "guest" {
 		return handleGuestRegistration(req, r, cfg, userAPI)
 	}
 
-	// Retrieve or generate the sessionID
-	sessionID := r.Auth.Session
-	if sessionID == "" {
-		// Generate a new, random session ID
-		sessionID = util.RandomString(sessionIDLength)
-	}
-
 	// Don't allow numeric usernames less than MAX_INT64.
 	if _, err := strconv.ParseInt(r.Username, 10, 64); err == nil {
 		return util.JSONResponse{
@@ -568,7 +584,7 @@ func Register(
 	case r.Type == authtypes.LoginTypeApplicationService && accessTokenErr == nil:
 		// Spec-compliant case (the access_token is specified and the login type
 		// is correctly set, so it's an appservice registration)
-		if resErr = validateApplicationServiceUsername(r.Username); resErr != nil {
+		if resErr := validateApplicationServiceUsername(r.Username); resErr != nil {
 			return *resErr
 		}
 	case accessTokenErr == nil:
@@ -581,11 +597,11 @@ func Register(
 	default:
 		// Spec-compliant case (neither the access_token nor the login type are
 		// specified, so it's a normal user registration)
-		if resErr = validateUsername(r.Username); resErr != nil {
+		if resErr := validateUsername(r.Username); resErr != nil {
 			return *resErr
 		}
 	}
-	if resErr = validatePassword(r.Password); resErr != nil {
+	if resErr := validatePassword(r.Password); resErr != nil {
 		return *resErr
 	}
 
@@ -835,24 +851,17 @@ func completeRegistration(
 		}
 	}()
 
-	if data, ok := sessions.getParams(sessionID); ok {
-		username = data.Username
-		password = data.Password
-		deviceID = data.DeviceID
-		displayName = data.InitialDisplayName
-		inhibitLogin = data.InhibitLogin
-	}
 	if username == "" {
 		return util.JSONResponse{
 			Code: http.StatusBadRequest,
-			JSON: jsonerror.BadJSON("missing username"),
+			JSON: jsonerror.MissingArgument("Missing username"),
 		}
 	}
 	// Blank passwords are only allowed by registered application services
 	if password == "" && appserviceID == "" {
 		return util.JSONResponse{
 			Code: http.StatusBadRequest,
-			JSON: jsonerror.BadJSON("missing password"),
+			JSON: jsonerror.MissingArgument("Missing password"),
 		}
 	}
 	var accRes userapi.PerformAccountCreationResponse