diff --git a/oauthproxy.go b/oauthproxy.go
index d11040c3..a1339415 100644
--- a/oauthproxy.go
+++ b/oauthproxy.go
@@ -919,7 +919,8 @@ func (p *OAuthProxy) enrichSessionState(ctx context.Context, s *sessionsapi.Sess
 // and optional authorization).
 func (p *OAuthProxy) AuthOnly(rw http.ResponseWriter, req *http.Request) {
 	session, err := p.getAuthenticatedSession(rw, req)
-	if err != nil {
+	if err != nil || session == nil {
+		// If there's no session, or an error retrieving it, we need to return 401 to trigger the OAuth2 flow.
 		http.Error(rw, http.StatusText(http.StatusUnauthorized), http.StatusUnauthorized)
 		return
 	}
diff --git a/oauthproxy_test.go b/oauthproxy_test.go
index 0d8bc91a..0eac9dab 100644
--- a/oauthproxy_test.go
+++ b/oauthproxy_test.go
@@ -1401,6 +1401,24 @@ func TestAuthOnlyEndpointSetBasicAuthFalseRequestHeaders(t *testing.T) {
 	assert.Equal(t, 0, len(pcTest.rw.Header().Values("Authorization")), "should not have Authorization header entries")
 }
 
+func TestAuthOnlyEndpointRejectPreflighRequests(t *testing.T) {
+	skipPreflight := func(opts *options.Options) {
+		opts.SkipAuthPreflight = true
+	}
+
+	test, err := NewAuthOnlyEndpointTest("", skipPreflight)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	test.req.Method = http.MethodOptions
+
+	test.proxy.ServeHTTP(test.rw, test.req)
+	assert.Equal(t, http.StatusUnauthorized, test.rw.Code)
+	bodyBytes, _ := io.ReadAll(test.rw.Body)
+	assert.Equal(t, "Unauthorized\n", string(bodyBytes))
+}
+
 func TestAuthSkippedForPreflightRequests(t *testing.T) {
 	upstreamServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(200)
@@ -3057,14 +3075,14 @@ func TestAuthOnlyAllowedGroupsWithSkipMethods(t *testing.T) {
 			method:             "OPTIONS",
 			ip:                 "1.2.3.5:43670",
 			withSession:        false,
-			expectedStatusCode: http.StatusAccepted,
+			expectedStatusCode: http.StatusUnauthorized,
 		},
 		{
 			name:               "UserWithoutSessionTrustedIp",
 			method:             "GET",
 			ip:                 "1.2.3.4:43670",
 			withSession:        false,
-			expectedStatusCode: http.StatusAccepted,
+			expectedStatusCode: http.StatusUnauthorized,
 		},
 	}