Index: src/wp-includes/formatting.php
===================================================================
--- src/wp-includes/formatting.php	(revision 57055)
+++ src/wp-includes/formatting.php	(working copy)
@@ -4961,7 +4961,12 @@
 			break;
 
 		case 'ping_sites':
-			$value = explode( "\n", $value );
+			if ( is_string( $value ) ) {
+				$value = explode( "\n", $value );
+			} else {
+				$value = array();
+			}
+
 			$value = array_filter( array_map( 'trim', $value ) );
 			$value = array_filter( array_map( 'sanitize_url', $value ) );
 			$value = implode( "\n", $value );
Index: tests/phpunit/tests/option/sanitizeOption.php
===================================================================
--- tests/phpunit/tests/option/sanitizeOption.php	(revision 57055)
+++ tests/phpunit/tests/option/sanitizeOption.php	(working copy)
@@ -44,6 +44,7 @@
 			array( 'date_format', 'F j, Y', 'F j, <strong>Y</strong>' ),
 			array( 'ping_sites', 'http://rpc.pingomatic.com/', 'http://rpc.pingomatic.com/' ),
 			array( 'ping_sites', "http://www.example.com\nhttp://example.org", "www.example.com \n\texample.org\n\n" ),
+			array( 'ping_sites', '', null ),
 			array( 'gmt_offset', '0', 0 ),
 			array( 'gmt_offset', '1.5', '1.5' ),
 			array( 'gmt_offset', '', null ),