diff --git a/.travis.yml b/.travis.yml
index 7ffcc4a..549910f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,7 +3,7 @@ osx_image: xcode7.2
 
 before_install:
 - brew update
-- brew install carthage
+- brew outdated carthage || brew upgrade carthage
 - if brew outdated | grep -qx xctool; then brew upgrade xctool; fi
 - carthage update --platform iOS
 - gem install xcpretty --no-rdoc --no-ri --no-document --quiet
diff --git a/Sources/ButtonBarPagerTabStripViewController.swift b/Sources/ButtonBarPagerTabStripViewController.swift
index 2bad00d..6b71aff 100644
--- a/Sources/ButtonBarPagerTabStripViewController.swift
+++ b/Sources/ButtonBarPagerTabStripViewController.swift
@@ -240,8 +240,8 @@ public class ButtonBarPagerTabStripViewController: PagerTabStripViewController,
         buttonBarView.moveToIndex(indexPath.item, animated: true, swipeDirection: .None, pagerScroll: .Yes)
         shouldUpdateButtonBarView = false
         
-        let oldCell = buttonBarView.cellForItemAtIndexPath(NSIndexPath(forItem: currentIndex, inSection: 0)) as! ButtonBarViewCell
-        let newCell = buttonBarView.cellForItemAtIndexPath(NSIndexPath(forItem: indexPath.item, inSection: 0)) as! ButtonBarViewCell
+        let oldCell = buttonBarView.cellForItemAtIndexPath(NSIndexPath(forItem: currentIndex, inSection: 0)) as? ButtonBarViewCell
+        let newCell = buttonBarView.cellForItemAtIndexPath(NSIndexPath(forItem: indexPath.item, inSection: 0)) as? ButtonBarViewCell
         if pagerBehaviour.isProgressiveIndicator {
             if let changeCurrentIndexProgressive = changeCurrentIndexProgressive {
                 changeCurrentIndexProgressive(oldCell: oldCell, newCell: newCell, progressPercentage: 1, changeCurrentIndex: true, animated: true)