diff --git a/src/wp-includes/Text/Diff.php b/src/wp-includes/Text/Diff.php
index eee4e4f8ea531..b3c63afc5f859 100644
--- a/src/wp-includes/Text/Diff.php
+++ b/src/wp-includes/Text/Diff.php
@@ -276,7 +276,7 @@ function _check($from_lines, $to_lines)
 
         $prevtype = null;
         foreach ($this->_edits as $edit) {
-            if ($edit instanceof $prevtype) {
+            if ($prevtype !== null && $edit instanceof $prevtype) {
                 trigger_error("Edit sequence is non-optimal", E_USER_ERROR);
             }
             $prevtype = get_class($edit);
diff --git a/tests/phpunit/tests/diff/Text_Diff_Check_Test.php b/tests/phpunit/tests/diff/Text_Diff_Check_Test.php
new file mode 100644
index 0000000000000..dec8193ad0896
--- /dev/null
+++ b/tests/phpunit/tests/diff/Text_Diff_Check_Test.php
@@ -0,0 +1,37 @@
+<?php
+
+/**
+ * Tests for WP native customizations added to the Text_Diff::check() method.
+ *
+ * @group diff
+ *
+ * @covers Text_Diff::_check
+ */
+final class Text_Diff_Check_Test extends WP_UnitTestCase {
+
+	const FILE_A = array(
+		'Line 1',
+		'Line 2',
+		'Line 3',
+	);
+
+	const FILE_B = array(
+		'Line 11',
+		'Line 2',
+		'Line 13',
+	);
+
+	public static function set_up_before_class() {
+		require_once ABSPATH . 'wp-includes/Text/Diff.php';
+	}
+
+	/**
+	 * Disable WP specific set up as it is not needed.
+	 */
+	public function set_up() {}
+
+	public function test_check_passes_when_passed_same_input() {
+		$diff = new Text_Diff( 'auto', array( self::FILE_A, self::FILE_B ) );
+		$this->assertTrue( $diff->_check( self::FILE_A, self::FILE_B ) );
+	}
+}