diff --git a/src/PhpSpreadsheet/Reader/Csv.php b/src/PhpSpreadsheet/Reader/Csv.php
index a142ef27..a8567bea 100644
--- a/src/PhpSpreadsheet/Reader/Csv.php
+++ b/src/PhpSpreadsheet/Reader/Csv.php
@@ -369,7 +369,7 @@ class Csv extends BaseReader
         return $this->delimiter;
     }
 
-    public function setDelimiter(string $delimiter): self
+    public function setDelimiter(?string $delimiter): self
     {
         $this->delimiter = $delimiter;
 
diff --git a/tests/PhpSpreadsheetTests/Reader/Csv/CsvTest.php b/tests/PhpSpreadsheetTests/Reader/Csv/CsvTest.php
index b29655fb..24887f1b 100644
--- a/tests/PhpSpreadsheetTests/Reader/Csv/CsvTest.php
+++ b/tests/PhpSpreadsheetTests/Reader/Csv/CsvTest.php
@@ -251,4 +251,25 @@ EOF;
             [(version_compare(PHP_VERSION, '7.4') < 0) ? "\x0" : '', ','],
         ];
     }
+
+    /**
+     * This test could be simpler, but Scrutinizer has a minor (and silly) problem.
+     *
+     * @dataProvider providerNull
+     */
+    public function testSetDelimiterNull(?string $setNull): void
+    {
+        $reader = new Csv();
+        $reader->setDelimiter(',');
+        self::assertSame(',', $reader->getDelimiter());
+        $reader->setDelimiter($setNull);
+        self::assertSame($setNull, $reader->getDelimiter());
+    }
+
+    public function providerNull(): array
+    {
+        return [
+            [null],
+        ];
+    }
 }