Ticket #24873: 28873.3.diff

wp-admin/includes/revision.php

@@ -179 +179 @@
         // Now, grab the initial diff
         $compare_two_mode = is_numeric( $from );
         if ( ! $compare_two_mode ) {
-                $from = array_keys( array_slice( $revisions, array_search( $selected_revision_id, array_keys( $revisions ) ) - 1, 1, true ) );
-                $from = $from[0];
+                $found = array_search( $selected_revision_id, array_keys( $revisions ) );
+                if ( $found )
+                        $from = array_shift( array_keys( array_slice( $revisions, $found - 1, 1, true ) ) );
+                else
+                        $from = 0;
         }
 
         $from = absint( $from );