diff --git a/src/PhpSpreadsheet/Calculation/CalculationBase.php b/src/PhpSpreadsheet/Calculation/CalculationBase.php
index 9dc67fbc2..5bef04d7e 100644
--- a/src/PhpSpreadsheet/Calculation/CalculationBase.php
+++ b/src/PhpSpreadsheet/Calculation/CalculationBase.php
@@ -30,7 +30,10 @@ class CalculationBase
     public static function addFunction(string $key, array $value): bool
     {
         $key = strtoupper($key);
-        if (array_key_exists($key, FunctionArray::$phpSpreadsheetFunctions)) {
+        if (
+            array_key_exists($key, FunctionArray::$phpSpreadsheetFunctions)
+            && !self::isDummy($key)
+        ) {
             return false;
         }
         $value['custom'] = true;
@@ -39,6 +42,20 @@ class CalculationBase
         return true;
     }
 
+    private static function isDummy(string $key): bool
+    {
+        // key is already known to exist
+        $functionCall = FunctionArray::$phpSpreadsheetFunctions[$key]['functionCall'] ?? null;
+        if (!is_array($functionCall)) {
+            return false;
+        }
+        if (($functionCall[1] ?? '') !== 'DUMMY') {
+            return false;
+        }
+
+        return true;
+    }
+
     public static function removeFunction(string $key): bool
     {
         $key = strtoupper($key);
diff --git a/tests/PhpSpreadsheetTests/Calculation/CustomFunction.php b/tests/PhpSpreadsheetTests/Calculation/CustomFunction.php
index 1ce5cf5d2..7e0fea0a7 100644
--- a/tests/PhpSpreadsheetTests/Calculation/CustomFunction.php
+++ b/tests/PhpSpreadsheetTests/Calculation/CustomFunction.php
@@ -6,6 +6,7 @@ namespace PhpOffice\PhpSpreadsheetTests\Calculation;
 
 use PhpOffice\PhpSpreadsheet\Calculation\Exception as CalcException;
 use PhpOffice\PhpSpreadsheet\Calculation\MathTrig\Helpers;
+use PhpOffice\PhpSpreadsheet\Shared\StringHelper;
 
 class CustomFunction
 {
@@ -19,4 +20,25 @@ class CustomFunction
 
         return $number ** 4;
     }
+
+    /**
+     * ASC.
+     * Converts full-width (double-byte) characters to half-width (single-byte) characters.
+     * There are many difficulties with implementing this into PhpSpreadsheet.
+     */
+    public static function ASC(mixed $stringValue): string
+    {
+        /*if (is_array($stringValue)) {
+            return self::evaluateSingleArgumentArray([self::class, __FUNCTION__], $stringValue);
+        }*/
+        $stringValue = StringHelper::convertToString($stringValue, convertBool: true);
+        if (function_exists('mb_convert_kana')) {
+            return mb_convert_kana($stringValue, 'a', 'UTF-8');
+        }
+        // Fallback if mb_convert_kana is not available.
+        // PhpSpreadsheet heavily relies on mbstring, so this is more of a theoretical fallback.
+        // A comprehensive manual conversion is extensive.
+
+        return $stringValue;
+    }
 }
diff --git a/tests/PhpSpreadsheetTests/Calculation/CustomFunctionTest.php b/tests/PhpSpreadsheetTests/Calculation/CustomFunctionTest.php
index 1c536fad4..03262dfc0 100644
--- a/tests/PhpSpreadsheetTests/Calculation/CustomFunctionTest.php
+++ b/tests/PhpSpreadsheetTests/Calculation/CustomFunctionTest.php
@@ -31,4 +31,19 @@ class CustomFunctionTest extends TestCase
         self::assertSame('#NAME?', $calculation->calculateFormula('=FOURTHPOWER(3)'));
         self::assertFalse(Calculation::removeFunction('WHATEVER'));
     }
+
+    public static function testReplaceDummyFunction(): void
+    {
+        $functions = Calculation::getFunctions();
+        $key = 'ASC';
+        $oldValue = $functions[$key] ?? null;
+        self::assertIsArray($oldValue);
+        $calculation = Calculation::getInstance();
+        $value = $oldValue;
+        $value['functionCall'] = [CustomFunction::class, 'ASC'];
+        self::assertTrue(Calculation::addFunction($key, $value));
+        self::assertSame('ABC', $calculation->calculateFormula('=ASC("ＡＢＣ")'));
+        self::assertTrue(Calculation::removeFunction('ASC'));
+        self::assertTrue(Calculation::addFunction($key, $oldValue));
+    }
 }