diff --git a/src/Tokenizers/PHP.php b/src/Tokenizers/PHP.php
index 9c6c11e4c3..c6b8e29065 100644
--- a/src/Tokenizers/PHP.php
+++ b/src/Tokenizers/PHP.php
@@ -2817,7 +2817,10 @@ protected function processAdditional()
                     }
                 }
 
-                if (isset($this->tokens[$x]) === true && $this->tokens[$x]['code'] === T_OPEN_PARENTHESIS) {
+                if (isset($this->tokens[$x]) === true
+                    && $this->tokens[$x]['code'] === T_OPEN_PARENTHESIS
+                    && isset($this->tokens[$x]['parenthesis_closer']) === true
+                ) {
                     $ignore  = Tokens::$emptyTokens;
                     $ignore += [
                         T_ARRAY                  => T_ARRAY,
@@ -2995,7 +2998,7 @@ protected function processAdditional()
                     }//end if
                 }//end if
 
-                // If after all that, the extra tokens are not set, this is not an arrow function.
+                // If after all that, the extra tokens are not set, this is not a (valid) arrow function.
                 if (isset($this->tokens[$i]['scope_closer']) === false) {
                     if (PHP_CODESNIFFER_VERBOSITY > 1) {
                         $line = $this->tokens[$i]['line'];
diff --git a/tests/Core/Tokenizers/PHP/BackfillFnTokenParseErrorTest.inc b/tests/Core/Tokenizers/PHP/BackfillFnTokenParseErrorTest.inc
new file mode 100644
index 0000000000..ce247e991b
--- /dev/null
+++ b/tests/Core/Tokenizers/PHP/BackfillFnTokenParseErrorTest.inc
@@ -0,0 +1,5 @@
+<?php
+
+/* testLiveCoding */
+// Intentional parse error. This has to be the only test in the file.
+$fn = static fn (string $paramA
diff --git a/tests/Core/Tokenizers/PHP/BackfillFnTokenParseErrorTest.php b/tests/Core/Tokenizers/PHP/BackfillFnTokenParseErrorTest.php
new file mode 100644
index 0000000000..7b762e8df2
--- /dev/null
+++ b/tests/Core/Tokenizers/PHP/BackfillFnTokenParseErrorTest.php
@@ -0,0 +1,44 @@
+<?php
+/**
+ * Tests the backfilling of the T_FN token to PHP < 7.4 for a specific parse error.
+ *
+ * @author    Juliette Reinders Folmer <phpcs_nospam@adviesenzo.nl>
+ * @copyright 2024 PHPCSStandards and contributors
+ * @license   https://github.com/PHPCSStandards/PHP_CodeSniffer/blob/master/licence.txt BSD Licence
+ */
+
+namespace PHP_CodeSniffer\Tests\Core\Tokenizers\PHP;
+
+use PHP_CodeSniffer\Tests\Core\Tokenizers\AbstractTokenizerTestCase;
+
+final class BackfillFnTokenParseErrorTest extends AbstractTokenizerTestCase
+{
+
+
+    /**
+     * Verify that un unfinished arrow function during live coding doesn't cause a "Undefined array key "parenthesis_closer"" error.
+     *
+     * @covers PHP_CodeSniffer\Tokenizers\PHP::processAdditional
+     *
+     * @return void
+     */
+    public function testUnfinishedArrowFunction()
+    {
+        $tokens = $this->phpcsFile->getTokens();
+
+        $token      = $this->getTargetToken('/* testLiveCoding */', [T_STRING, T_FN], 'fn');
+        $tokenArray = $tokens[$token];
+
+        $this->assertSame('T_STRING', $tokenArray['type'], 'Token tokenized as '.$tokenArray['type'].', not T_STRING');
+
+        $this->assertArrayNotHasKey('scope_condition', $tokenArray, 'Scope condition is set');
+        $this->assertArrayNotHasKey('scope_opener', $tokenArray, 'Scope opener is set');
+        $this->assertArrayNotHasKey('scope_closer', $tokenArray, 'Scope closer is set');
+        $this->assertArrayNotHasKey('parenthesis_owner', $tokenArray, 'Parenthesis owner is set');
+        $this->assertArrayNotHasKey('parenthesis_opener', $tokenArray, 'Parenthesis opener is set');
+        $this->assertArrayNotHasKey('parenthesis_closer', $tokenArray, 'Parenthesis closer is set');
+
+    }//end testUnfinishedArrowFunction()
+
+
+}//end class