diff --git a/src/Entity/InvoiceTemplate.php b/src/Entity/InvoiceTemplate.php
index 78d3a36b..4b97f636 100644
--- a/src/Entity/InvoiceTemplate.php
+++ b/src/Entity/InvoiceTemplate.php
@@ -306,7 +306,7 @@ class InvoiceTemplate
      * @param string $paymentTerms
      * @return InvoiceTemplate
      */
-    public function setPaymentTerms(string $paymentTerms)
+    public function setPaymentTerms(?string $paymentTerms)
     {
         $this->paymentTerms = $paymentTerms;
 
diff --git a/tests/Entity/AbstractEntityTest.php b/tests/Entity/AbstractEntityTest.php
new file mode 100644
index 00000000..74b60b09
--- /dev/null
+++ b/tests/Entity/AbstractEntityTest.php
@@ -0,0 +1,57 @@
+<?php
+
+/*
+ * This file is part of the Kimai time-tracking app.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace App\Tests\Entity;
+
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Validator\ConstraintViolationInterface;
+use Symfony\Component\Validator\Validation;
+
+/**
+ * @covers \App\Entity\Timesheet
+ */
+abstract class AbstractEntityTest extends TestCase
+{
+    /**
+     * @param $value
+     * @param array|string $fieldNames
+     */
+    protected function assertHasViolationForField($value, $fieldNames)
+    {
+        $validator = Validation::createValidatorBuilder()->enableAnnotationMapping()->getValidator();
+        $validations = $validator->validate($value);
+
+        if (!is_array($fieldNames)) {
+            $fieldNames = [$fieldNames];
+        }
+
+        $violatedFields = [];
+        /** @var ConstraintViolationInterface $validation */
+        foreach ($validations as $validation) {
+            $violatedFields[] = $validation->getPropertyPath();
+        }
+
+        foreach ($fieldNames as $id => $propertyPath) {
+            $foundField = false;
+            if (in_array($propertyPath, $violatedFields)) {
+                $foundField = true;
+                unset($violatedFields[$id]);
+            }
+
+            $this->assertTrue($foundField, 'Failed finding violation for field: ' . $propertyPath);
+        }
+
+        $this->assertEmpty($violatedFields, sprintf('Unexpected violations found: %s', implode(', ', $violatedFields)));
+
+        $expected = count($fieldNames);
+        $actual = $validations->count();
+
+        $this->assertEquals($expected, $actual, sprintf('Expected %s violations, found %s.', $expected, $actual));
+    }
+}
diff --git a/tests/Entity/InvoiceTemplateTest.php b/tests/Entity/InvoiceTemplateTest.php
new file mode 100644
index 00000000..94be52cf
--- /dev/null
+++ b/tests/Entity/InvoiceTemplateTest.php
@@ -0,0 +1,51 @@
+<?php
+
+/*
+ * This file is part of the Kimai time-tracking app.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace App\Tests\Entity;
+
+use App\Entity\InvoiceTemplate;
+
+/**
+ * @covers \App\Entity\InvoiceTemplate
+ */
+class InvoiceTemplateTest extends AbstractEntityTest
+{
+    protected function assertIsFluent($actual)
+    {
+        $this->assertInstanceOf(InvoiceTemplate::class, $actual);
+    }
+
+    protected function getEntity()
+    {
+        $entity = new InvoiceTemplate();
+
+        return $entity;
+    }
+
+    public function testSetGetPaymentTerms()
+    {
+        $sut = $this->getEntity();
+
+        $this->assertNull($sut->getPaymentTerms());
+        $this->assertIsFluent($sut->setPaymentTerms(null));
+        $this->assertIsFluent($sut->setPaymentTerms(''));
+        $this->assertIsFluent($sut->setPaymentTerms('foo bar'));
+        $this->assertEquals('foo bar', $sut->getPaymentTerms());
+    }
+
+    public function testToString()
+    {
+        $sut = $this->getEntity();
+
+        $this->assertNull($sut->__toString());
+        $this->assertIsFluent($sut->setName('a template name'));
+        $this->assertEquals('a template name', $sut->__toString());
+        $this->assertEquals('a template name', (string) $sut);
+    }
+}
diff --git a/tests/Entity/TimesheetTest.php b/tests/Entity/TimesheetTest.php
index 3c9a751e..21b4bb30 100644
--- a/tests/Entity/TimesheetTest.php
+++ b/tests/Entity/TimesheetTest.php
@@ -7,21 +7,18 @@
  * file that was distributed with this source code.
  */
 
-namespace App\Tests\Twig;
+namespace App\Tests\Entity;
 
 use App\Entity\Activity;
 use App\Entity\Customer;
 use App\Entity\Project;
 use App\Entity\Timesheet;
 use App\Entity\User;
-use PHPUnit\Framework\TestCase;
-use Symfony\Component\Validator\ConstraintViolationInterface;
-use Symfony\Component\Validator\Validation;
 
 /**
  * @covers \App\Entity\Timesheet
  */
-class TimesheetTest extends TestCase
+class TimesheetTest extends AbstractEntityTest
 {
     protected function getEntity()
     {
@@ -85,41 +82,4 @@ class TimesheetTest extends TestCase
 
         $this->assertHasViolationForField($entity, []);
     }
-
-    /**
-     * @param $value
-     * @param array|string $fieldNames
-     */
-    protected function assertHasViolationForField($value, $fieldNames)
-    {
-        $validator = Validation::createValidatorBuilder()->enableAnnotationMapping()->getValidator();
-        $validations = $validator->validate($value);
-
-        if (!is_array($fieldNames)) {
-            $fieldNames = [$fieldNames];
-        }
-
-        $violatedFields = [];
-        /** @var ConstraintViolationInterface $validation */
-        foreach ($validations as $validation) {
-            $violatedFields[] = $validation->getPropertyPath();
-        }
-
-        foreach ($fieldNames as $id => $propertyPath) {
-            $foundField = false;
-            if (in_array($propertyPath, $violatedFields)) {
-                $foundField = true;
-                unset($violatedFields[$id]);
-            }
-
-            $this->assertTrue($foundField, 'Failed finding violation for field: ' . $propertyPath);
-        }
-
-        $this->assertEmpty($violatedFields, sprintf('Unexpected violations found: %s', implode(', ', $violatedFields)));
-
-        $expected = count($fieldNames);
-        $actual = $validations->count();
-
-        $this->assertEquals($expected, $actual, sprintf('Expected %s violations, found %s.', $expected, $actual));
-    }
 }