diff --git a/src/PhpSpreadsheet/Calculation/Calculation.php b/src/PhpSpreadsheet/Calculation/Calculation.php
index f2e6647f..fad28dc9 100644
--- a/src/PhpSpreadsheet/Calculation/Calculation.php
+++ b/src/PhpSpreadsheet/Calculation/Calculation.php
@@ -3644,7 +3644,7 @@ class Calculation
                 } else {    // it's a variable, constant, string, number or boolean
                     //    If the last entry on the stack was a : operator, then we may have a row or column range reference
                     $testPrevOp = $stack->last(1);
-                    if ($testPrevOp['value'] == ':') {
+                    if ($testPrevOp !== null && $testPrevOp['value'] === ':') {
                         $startRowColRef = $output[count($output) - 1]['value'];
                         [$rangeWS1, $startRowColRef] = Worksheet::extractSheetTitle($startRowColRef, true);
                         if ($rangeWS1 != '') {
diff --git a/tests/PhpSpreadsheetTests/Custom/ComplexAssert.php b/tests/PhpSpreadsheetTests/Custom/ComplexAssert.php
index 762b2886..d039dab0 100644
--- a/tests/PhpSpreadsheetTests/Custom/ComplexAssert.php
+++ b/tests/PhpSpreadsheetTests/Custom/ComplexAssert.php
@@ -35,7 +35,7 @@ class ComplexAssert
 
     public function assertComplexEquals($expected, $actual, $delta = 0)
     {
-        if ($expected === INF || $expected[0] === '#') {
+        if ($expected === INF || (is_string($expected) && $expected[0] === '#')) {
             return $this->testExpectedExceptions($expected, $actual);
         }