diff --git a/src/wp-includes/class-wp-xmlrpc-server.php b/src/wp-includes/class-wp-xmlrpc-server.php
index 1fff1bba65adf..8b1ed095aff42 100644
--- a/src/wp-includes/class-wp-xmlrpc-server.php
+++ b/src/wp-includes/class-wp-xmlrpc-server.php
@@ -6448,7 +6448,11 @@ public function mw_getCategories( $args ) {
 	public function mw_newMediaObject( $args ) {
 		$username = $this->escape( $args[1] );
 		$password = $this->escape( $args[2] );
-		$data     = $args[3];
+		$data     = $args[3] ?? null;
+
+		if ( ! is_array( $data ) ) {
+			return new IXR_Error( 400, __( 'Invalid attachment data.' ) );
+		}
 
 		$name = sanitize_file_name( $data['name'] );
 		$type = $data['type'];
diff --git a/tests/phpunit/tests/xmlrpc/wp/uploadFile.php b/tests/phpunit/tests/xmlrpc/wp/uploadFile.php
index 00cb601b28f3d..7b466c1993b1b 100644
--- a/tests/phpunit/tests/xmlrpc/wp/uploadFile.php
+++ b/tests/phpunit/tests/xmlrpc/wp/uploadFile.php
@@ -34,4 +34,37 @@ public function test_valid_attachment() {
 		$this->assertIsString( $result['url'] );
 		$this->assertIsString( $result['type'] );
 	}
+
+	/**
+	 * Tests that a non-array data argument returns an error instead of
+	 * triggering a fatal error.
+	 *
+	 * The data argument (the fourth parameter) is expected to be a struct,
+	 * which is passed to the method as an array. When it is any other type,
+	 * the method must return an IXR_Error rather than attempting to access
+	 * array offsets on a non-array value.
+	 *
+	 * @ticket 65611
+	 */
+	public function test_invalid_attachment_data_should_return_error() {
+		$this->make_user_by_role( 'editor' );
+
+		$result = $this->myxmlrpcserver->mw_newMediaObject( array( 0, 'editor', 'editor', 'not-a-struct' ) );
+		$this->assertIXRError( $result, 'A non-array data argument should return an IXR_Error.' );
+		$this->assertSame( 400, $result->code, 'The error code should be 400.' );
+	}
+
+	/**
+	 * Tests that a missing data argument returns an error instead of
+	 * emitting a PHP notice for the undefined argument.
+	 *
+	 * @ticket 65611
+	 */
+	public function test_missing_attachment_data_should_return_error() {
+		$this->make_user_by_role( 'editor' );
+
+		$result = $this->myxmlrpcserver->mw_newMediaObject( array( 0, 'editor', 'editor' ) );
+		$this->assertIXRError( $result, 'A missing data argument should return an IXR_Error.' );
+		$this->assertSame( 400, $result->code, 'The error code should be 400.' );
+	}
 }