diff --git a/app/code/Magento/Customer/Model/Customer/Source/Group.php b/app/code/Magento/Customer/Model/Customer/Source/Group.php
index 98cba5dd555a9..e4c1d2e75be22 100644
--- a/app/code/Magento/Customer/Model/Customer/Source/Group.php
+++ b/app/code/Magento/Customer/Model/Customer/Source/Group.php
@@ -56,7 +56,7 @@ public function toOptionArray()
         $customerGroups = [];
         $customerGroups[] = [
             'label' => __('ALL GROUPS'),
-            'value' => GroupInterface::CUST_GROUP_ALL,
+            'value' => (string)GroupInterface::CUST_GROUP_ALL,
         ];
 
         /** @var GroupSearchResultsInterface $groups */
diff --git a/app/code/Magento/Customer/Test/Unit/Model/Customer/Source/GroupTest.php b/app/code/Magento/Customer/Test/Unit/Model/Customer/Source/GroupTest.php
index e93d81c45fda4..78148fac3a9a6 100644
--- a/app/code/Magento/Customer/Test/Unit/Model/Customer/Source/GroupTest.php
+++ b/app/code/Magento/Customer/Test/Unit/Model/Customer/Source/GroupTest.php
@@ -71,8 +71,8 @@ protected function setUp()
     public function testToOptionArray()
     {
         $customerGroups = [
-            ['label' => __('ALL GROUPS'), 'value' => 32000],
-            ['label' => __('NOT LOGGED IN'), 'value' => 0]
+            ['label' => __('ALL GROUPS'), 'value' => '32000'],
+            ['label' => __('NOT LOGGED IN'), 'value' => '0'],
         ];
 
         $this->moduleManagerMock->expects($this->any())
@@ -95,11 +95,17 @@ public function testToOptionArray()
             ->setMethods(['getCode', 'getId'])
             ->getMockForAbstractClass();
         $groupTest->expects($this->any())->method('getCode')->willReturn(__('NOT LOGGED IN'));
-        $groupTest->expects($this->any())->method('getId')->willReturn(0);
+        $groupTest->expects($this->any())->method('getId')->willReturn('0');
         $groups = [$groupTest];
 
         $this->searchResultMock->expects($this->any())->method('getItems')->willReturn($groups);
 
-        $this->assertEquals($customerGroups, $this->model->toOptionArray());
+        $actualCustomerGroups = $this->model->toOptionArray();
+
+        $this->assertEquals($customerGroups, $actualCustomerGroups);
+
+        foreach ($actualCustomerGroups as $actualCustomerGroup) {
+            $this->assertInternalType('string', $actualCustomerGroup['value']);
+        }
     }
 }