diff --git a/src/PhpSpreadsheet/Calculation/Calculation.php b/src/PhpSpreadsheet/Calculation/Calculation.php
index 0f8c1b97..2cf57915 100644
--- a/src/PhpSpreadsheet/Calculation/Calculation.php
+++ b/src/PhpSpreadsheet/Calculation/Calculation.php
@@ -3895,12 +3895,12 @@ class Calculation
         //        so we store the parent worksheet so that we can re-attach it when necessary
         $pCellParent = ($cell !== null) ? $cell->getWorksheet() : null;
 
-        $regexpMatchString = '/^(' . self::CALCULATION_REGEXP_FUNCTION .
+        $regexpMatchString = '/^(' . self::CALCULATION_REGEXP_STRING .
+                                '|' . self::CALCULATION_REGEXP_FUNCTION .
                                 '|' . self::CALCULATION_REGEXP_CELLREF .
                                 '|' . self::CALCULATION_REGEXP_COLUMN_RANGE .
                                 '|' . self::CALCULATION_REGEXP_ROW_RANGE .
                                 '|' . self::CALCULATION_REGEXP_NUMBER .
-                                '|' . self::CALCULATION_REGEXP_STRING .
                                 '|' . self::CALCULATION_REGEXP_OPENBRACE .
                                 '|' . self::CALCULATION_REGEXP_DEFINEDNAME .
                                 '|' . self::CALCULATION_REGEXP_ERROR .
diff --git a/tests/PhpSpreadsheetTests/Calculation/Functions/Information/IsRefTest.php b/tests/PhpSpreadsheetTests/Calculation/Functions/Information/IsRefTest.php
index 04d71959..d723cc08 100644
--- a/tests/PhpSpreadsheetTests/Calculation/Functions/Information/IsRefTest.php
+++ b/tests/PhpSpreadsheetTests/Calculation/Functions/Information/IsRefTest.php
@@ -23,8 +23,9 @@ class IsRefTest extends AllSetupTeardown
         $sheet->getCell('A8')->setValue('=ISREF(INDIRECT("' . $sheet->getTitle() . '" & "!" & "A1"))');
         $sheet->getCell('A9')->setValue('=ISREF(INDIRECT("A1"))');
         $sheet->getCell('A10')->setValue('=ISREF(INDIRECT("Invalid Worksheet" & "!" & "A1"))');
-        $sheet->getCell('A11')->setValue('=ISREF(ZZZ1)');
-        $sheet->getCell('A12')->setValue('=ISREF(CHOOSE(2, A1, B1, C1))');
+        $sheet->getCell('A11')->setValue('=ISREF(INDIRECT("Invalid Worksheet" & "!A1"))');
+        $sheet->getCell('A12')->setValue('=ISREF(ZZZ1)');
+        $sheet->getCell('A13')->setValue('=ISREF(CHOOSE(2, A1, B1, C1))');
 
         self::assertTrue($sheet->getCell('A1')->getCalculatedValue()); // Cell Reference
         self::assertTrue($sheet->getCell('A2')->getCalculatedValue()); // Cell Range
@@ -36,7 +37,8 @@ class IsRefTest extends AllSetupTeardown
         self::assertTrue($sheet->getCell('A8')->getCalculatedValue()); // Indirect to a Cell Reference
         self::assertTrue($sheet->getCell('A9')->getCalculatedValue()); // Indirect to a Worksheet/Cell Reference
         self::assertFalse($sheet->getCell('A10')->getCalculatedValue()); // Indirect to an Invalid Worksheet/Cell Reference
-        self::assertFalse($sheet->getCell('A11')->getCalculatedValue()); // Invalid Cell Reference
-        self::assertTrue($sheet->getCell('A12')->getCalculatedValue()); // returned Cell Reference
+        self::assertFalse($sheet->getCell('A11')->getCalculatedValue()); // Indirect to an Invalid Worksheet/Cell Reference
+        self::assertFalse($sheet->getCell('A12')->getCalculatedValue()); // Invalid Cell Reference
+        self::assertTrue($sheet->getCell('A13')->getCalculatedValue()); // returned Cell Reference
     }
 }