diff --git a/Sources/BaseButtonBarPagerTabStripViewController.swift b/Sources/BaseButtonBarPagerTabStripViewController.swift
index eb205d1..371718b 100644
--- a/Sources/BaseButtonBarPagerTabStripViewController.swift
+++ b/Sources/BaseButtonBarPagerTabStripViewController.swift
@@ -205,8 +205,8 @@ public class BaseButtonBarPagerTabStripViewController<ButtonBarCellType : UIColl
         buttonBarView.moveToIndex(indexPath.item, animated: true, swipeDirection: .None, pagerScroll: .Yes)
         shouldUpdateButtonBarView = false
         
-        let oldCell = buttonBarView.cellForItemAtIndexPath(NSIndexPath(forItem: currentIndex, inSection: 0)) as! ButtonBarCellType
-        let newCell = buttonBarView.cellForItemAtIndexPath(NSIndexPath(forItem: indexPath.item, inSection: 0)) as! ButtonBarCellType
+        let oldCell = buttonBarView.cellForItemAtIndexPath(NSIndexPath(forItem: currentIndex, inSection: 0)) as? ButtonBarCellType
+        let newCell = buttonBarView.cellForItemAtIndexPath(NSIndexPath(forItem: indexPath.item, inSection: 0)) as? ButtonBarCellType
         if pagerBehaviour.isProgressiveIndicator {
             if let changeCurrentIndexProgressive = changeCurrentIndexProgressive {
                 changeCurrentIndexProgressive(oldCell: oldCell, newCell: newCell, progressPercentage: 1, changeCurrentIndex: true, animated: true)