diff --git a/tests/PhpSpreadsheetTests/Calculation/CustomFunctionTest.php b/tests/PhpSpreadsheetTests/Calculation/CustomFunctionTest.php
index 5d9aa974b..454227aa4 100644
--- a/tests/PhpSpreadsheetTests/Calculation/CustomFunctionTest.php
+++ b/tests/PhpSpreadsheetTests/Calculation/CustomFunctionTest.php
@@ -39,6 +39,13 @@ class CustomFunctionTest extends TestCase
         self::assertTrue(Calculation::addFunction($key, $value));
         self::assertSame(1.0, $calculation->calculateFormula('=NATIVECOS(0)'));
         self::assertTrue(Calculation::removeFunction($key));
+        $key = 'PI';
+        $value = [
+            'category' => 'custom',
+            'functionCall' => 'pi',
+            'argumentCount' => '0',
+        ];
+        self::assertFalse(Calculation::addFunction($key, $value));
     }
 
     public static function testReplaceDummyFunction(): void