diff --git a/src/PhpSpreadsheet/Cell/Coordinate.php b/src/PhpSpreadsheet/Cell/Coordinate.php
index 506783979..b60691302 100644
--- a/src/PhpSpreadsheet/Cell/Coordinate.php
+++ b/src/PhpSpreadsheet/Cell/Coordinate.php
@@ -260,13 +260,13 @@ abstract class Coordinate
     }
 
     /**
-     * Column index from string.
+     * Return a column index number from a string column address.
      *
-     * @param string $columnAddress eg 'A'
+     * @param string $columnAddress eg 'A', 'AZ', 'BA' or 'IV'
      *
-     * @return int Column index (A = 1)
+     * @return int The column index, offset from 1. (A = 1, Az = 52, BA = 53, IV = 256).
      */
-    public static function columnIndexFromString($columnAddress)
+    public static function columnIndexFromString(string $columnAddress): int
     {
         //    Using a lookup cache adds a slight memory overhead, but boosts speed
         //    caching using a static within the method is faster than a class static,
@@ -291,6 +291,10 @@ abstract class Coordinate
         //    We also use the language construct isset() rather than the more costly strlen() function to match the
         //       length of $columnAddress for improved performance
         if (isset($columnAddress[0])) {
+            if (ctype_alpha($columnAddress) === false) {
+                throw new Exception('Column string address must be alpha characters only');
+            }
+
             if (!isset($columnAddress[1])) {
                 $indexCache[$columnAddress] = $columnLookup[$columnAddress];
 
@@ -310,22 +314,28 @@ abstract class Coordinate
         }
 
         throw new Exception(
-            'Column string index can not be ' . ((isset($columnAddress[0])) ? 'longer than 3 characters' : 'empty')
+            'Column string address can not be ' . ((isset($columnAddress[0])) ? 'longer than 3 characters' : 'empty')
         );
     }
 
     /**
-     * String from column index.
+     * Return a string column address from a column index number.
      *
-     * @param int $columnIndex Column index (A = 1)
+     * @param int $columnIndex the column index, This should always be a positive integer
      *
-     * @return string
+     * @return  string The string column address for the specified index. eg (A = 1, Az = 52, BA = 53, IV = 256).
      */
-    public static function stringFromColumnIndex($columnIndex)
+    public static function stringFromColumnIndex(int $columnIndex): string
     {
         static $indexCache = [];
         static $lookupCache = ' ABCDEFGHIJKLMNOPQRSTUVWXYZ';
 
+        if ($columnIndex <= 0) {
+            var_dump($columnIndex);
+
+            throw new Exception('Column index must be a positive integer');
+        }
+
         if (!isset($indexCache[$columnIndex])) {
             $indexValue = $columnIndex;
             $base26 = '';
diff --git a/src/PhpSpreadsheet/Reader/Xlsx.php b/src/PhpSpreadsheet/Reader/Xlsx.php
index 52df94e4c..6beb7a021 100644
--- a/src/PhpSpreadsheet/Reader/Xlsx.php
+++ b/src/PhpSpreadsheet/Reader/Xlsx.php
@@ -280,7 +280,9 @@ class Xlsx extends BaseReader
                         $xml->close();
 
                         $tmpInfo['lastColumnIndex'] = $tmpInfo['totalColumns'] - 1;
-                        $tmpInfo['lastColumnLetter'] = Coordinate::stringFromColumnIndex($tmpInfo['lastColumnIndex'] + 1);
+                        $tmpInfo['lastColumnLetter'] = ($tmpInfo['lastColumnIndex'] < 0)
+                            ? null
+                            : Coordinate::stringFromColumnIndex($tmpInfo['lastColumnIndex'] + 1);
 
                         $worksheetInfo[] = $tmpInfo;
                     }
diff --git a/tests/PhpSpreadsheetTests/Cell/CoordinateTest.php b/tests/PhpSpreadsheetTests/Cell/CoordinateTest.php
index 8ed234279..283383fa5 100644
--- a/tests/PhpSpreadsheetTests/Cell/CoordinateTest.php
+++ b/tests/PhpSpreadsheetTests/Cell/CoordinateTest.php
@@ -37,7 +37,7 @@ class CoordinateTest extends TestCase
             Coordinate::columnIndexFromString($cellAddress);
         } catch (\Exception $e) {
             self::assertInstanceOf(Exception::class, $e);
-            self::assertEquals($e->getMessage(), 'Column string index can not be longer than 3 characters');
+            self::assertEquals($e->getMessage(), 'Column string address can not be longer than 3 characters');
 
             return;
         }
@@ -52,7 +52,22 @@ class CoordinateTest extends TestCase
             Coordinate::columnIndexFromString($cellAddress);
         } catch (\Exception $e) {
             self::assertInstanceOf(Exception::class, $e);
-            self::assertEquals($e->getMessage(), 'Column string index can not be empty');
+            self::assertEquals($e->getMessage(), 'Column string address can not be empty');
+
+            return;
+        }
+        self::fail('An expected exception has not been raised.');
+    }
+
+    public function testColumnIndexFromInvalidString(): void
+    {
+        $cellAddress = '1';
+
+        try {
+            Coordinate::columnIndexFromString($cellAddress);
+        } catch (\Exception $e) {
+            self::assertInstanceOf(Exception::class, $e);
+            self::assertEquals($e->getMessage(), 'Column string address must be alpha characters only');
 
             return;
         }
@@ -79,6 +94,21 @@ class CoordinateTest extends TestCase
         return require 'tests/data/ColumnIndex.php';
     }
 
+    public function testStringFromInvalidColumnIndex(): void
+    {
+        $columnIndex = -1;
+
+        try {
+            $string = Coordinate::stringFromColumnIndex($columnIndex);
+        } catch (\Exception $e) {
+            self::assertInstanceOf(Exception::class, $e);
+            self::assertEquals($e->getMessage(), 'Column index must be a positive integer');
+
+            return;
+        }
+        self::fail('An expected exception has not been raised.');
+    }
+
     /**
      * @dataProvider providerCoordinates
      *
diff --git a/tests/PhpSpreadsheetTests/Reader/Xlsx/WorksheetInfoNamesTest.php b/tests/PhpSpreadsheetTests/Reader/Xlsx/WorksheetInfoNamesTest.php
index cc2269b2a..4e4a83546 100644
--- a/tests/PhpSpreadsheetTests/Reader/Xlsx/WorksheetInfoNamesTest.php
+++ b/tests/PhpSpreadsheetTests/Reader/Xlsx/WorksheetInfoNamesTest.php
@@ -99,6 +99,7 @@ class WorksheetInfoNamesTest extends TestCase
 
         self::assertSame('Chart1', $chartSheetInfo['worksheetName']);
         self::assertSame(-1, $chartSheetInfo['lastColumnIndex']);
+        self::assertNull($chartSheetInfo['lastColumnLetter']);
         self::assertSame(0, $chartSheetInfo['totalRows']);
         self::assertSame(0, $chartSheetInfo['totalColumns']);
     }
diff --git a/tests/data/ColumnIndex.php b/tests/data/ColumnIndex.php
index 750ef823f..3f585b0c6 100644
--- a/tests/data/ColumnIndex.php
+++ b/tests/data/ColumnIndex.php
@@ -25,6 +25,10 @@ return [
         'BA',
         53,
     ],
+    [
+        'BB',
+        54,
+    ],
     [
         'BZ',
         78,
@@ -49,4 +53,8 @@ return [
         'BAA',
         1379,
     ],
+    [
+        'XFD',
+        16384,
+    ],
 ];
diff --git a/tests/data/ColumnString.php b/tests/data/ColumnString.php
index 78d21aaa7..2746162bc 100644
--- a/tests/data/ColumnString.php
+++ b/tests/data/ColumnString.php
@@ -25,6 +25,10 @@ return [
         53,
         'BA',
     ],
+    [
+        54,
+        'BB',
+    ],
     [
         78,
         'BZ',
@@ -49,4 +53,8 @@ return [
         1379,
         'BAA',
     ],
+    [
+        16384,
+        'XFD',
+    ],
 ];