diff --git a/Sources/TwitterPagerTabStripViewController.swift b/Sources/TwitterPagerTabStripViewController.swift
index e8093b4..513268b 100644
--- a/Sources/TwitterPagerTabStripViewController.swift
+++ b/Sources/TwitterPagerTabStripViewController.swift
@@ -87,7 +87,7 @@ open class TwitterPagerTabStripViewController: PagerTabStripViewController, Page
     open func updateIndicator(for viewController: PagerTabStripViewController, fromIndex: Int, toIndex: Int, withProgressPercentage progressPercentage: CGFloat, indexWasChanged: Bool) {
 
         // move indicator scroll view
-        let distance = distanceValue
+        guard let distance = distanceValue else { return }
         var xOffset: CGFloat = 0
         if fromIndex < toIndex {
             xOffset = distance * CGFloat(fromIndex) + distance * progressPercentage
@@ -193,7 +193,7 @@ open class TwitterPagerTabStripViewController: PagerTabStripViewController, Page
     }
     
     private func setNavigationViewItemsPosition(updateAlpha: Bool) {
-        let distance = distanceValue
+        guard let distance = distanceValue else { return }
         let isPortrait = UIApplication.shared.statusBarOrientation.isPortrait
         let navBarHeight: CGFloat = navigationController!.navigationBar.frame.size.height
         for (index, label) in childTitleLabels.enumerated() {
@@ -231,8 +231,7 @@ open class TwitterPagerTabStripViewController: PagerTabStripViewController, Page
         }
     }
     
-    private var distanceValue: CGFloat {
-        let middle = navigationController!.navigationBar.convert(navigationController!.navigationBar.center, to: titleView)
-        return middle.x
+    private var distanceValue: CGFloat? {
+        return navigationController.map { $0.navigationBar.convert($0.navigationBar.center, to: titleView) }?.x
     }
 }