Index: wp-admin/includes/revision.php
===================================================================
--- wp-admin/includes/revision.php	(revision 24862)
+++ wp-admin/includes/revision.php	(working copy)
@@ -175,12 +175,15 @@
 		}
 		$revisions[ $current_id ]['current'] = true;
 	}
-
 	// 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];
+		// Check position of selected revision, if at 0 $from should be 0
+		$selected_revision_position = array_search( $selected_revision_id, array_keys( $revisions ) );		
+		if ( 0 !== $selected_revision_position ) 
+			$from = array_keys( array_slice( $revisions, $selected_revision_position - 1, 1, true ) )[0];
+		else
+			$from = 0;
 	}
 
 	$from = absint( $from );
Index: wp-admin/js/revisions.js
===================================================================
--- wp-admin/js/revisions.js	(revision 24862)
+++ wp-admin/js/revisions.js	(working copy)
@@ -656,6 +656,8 @@
 			_(tickCount).times( function( index ){
 				this.$el.append( '<div style="' + this.direction + ': ' + ( 100 * tickWidth * index ) + '%"></div>' );
 			}, this );
+
+			this.reportTickPosition();
 		}
 	});