diff --git a/src/Eccube/Entity/Member.php b/src/Eccube/Entity/Member.php
index ba2bc1edd82..18127b3ebe9 100644
--- a/src/Eccube/Entity/Member.php
+++ b/src/Eccube/Entity/Member.php
@@ -287,7 +287,7 @@ public function getPlainPassword(): ?string
          *
          * @return $this
          */
-        public function setPlainPassword(string $password): self
+        public function setPlainPassword(?string $password): self
         {
             $this->plainPassword = $password;
 
diff --git a/tests/Eccube/Tests/Web/Admin/Setting/System/MemberControllerTest.php b/tests/Eccube/Tests/Web/Admin/Setting/System/MemberControllerTest.php
index a2f34fa6eed..d3a6bdd8181 100644
--- a/tests/Eccube/Tests/Web/Admin/Setting/System/MemberControllerTest.php
+++ b/tests/Eccube/Tests/Web/Admin/Setting/System/MemberControllerTest.php
@@ -222,6 +222,32 @@ public function testMemberEditSubmitFail()
         $this->assertTrue($this->client->getResponse()->isSuccessful());
     }
 
+    /**
+     * @see https://github.com/EC-CUBE/ec-cube/issues/5420
+     */
+    public function testMemberEditSubmitFailWithPlainPasswordIsEmpty()
+    {
+        // before
+        $formData = $this->createFormData();
+        $formData['plain_password'] = [
+            'first' => '',
+            'second' => ''
+        ];
+        $Member = $this->createMember();
+        $Member->setPassword('**********');
+        $this->entityManager->persist($Member);
+        $this->entityManager->flush();
+        $mid = $Member->getId();
+
+        // main
+        $this->client->request('POST',
+            $this->generateUrl('admin_setting_system_member_edit', ['id' => $mid]),
+            ['admin_member' => $formData]
+        );
+
+        $this->assertTrue($this->client->getResponse()->isSuccessful());
+    }
+
     public function testMemberUpNotFoundMember()
     {
         // before