Index: wp-includes/option.php
===================================================================
--- wp-includes/option.php	(revision 23840)
+++ wp-includes/option.php	(working copy)
@@ -458,7 +458,7 @@
 			$alloptions = wp_load_alloptions();
 			if ( !isset( $alloptions[$transient_option] ) ) {
 				$transient_timeout = '_transient_timeout_' . $transient;
-				if ( get_option( $transient_timeout ) < time() ) {
+				if ( false !== get_option( $transient_timeout ) && get_option( $transient_timeout ) < time() ) {
 					delete_option( $transient_option  );
 					delete_option( $transient_timeout );
 					return false;