diff --git a/lib/sheet_viewer_page.dart b/lib/sheet_viewer_page.dart
index 9c59c0b..cca5e70 100644
--- a/lib/sheet_viewer_page.dart
+++ b/lib/sheet_viewer_page.dart
@@ -291,7 +291,7 @@ class PaintablePages extends StatelessWidget {
 class Pages extends StatelessWidget {
   final PdfDocument document;
   final int numPages;
-  final int currentPageNumber;
+  final int currentPageNumber; // Starts at 1
   final Config config;
 
   const Pages({
@@ -362,7 +362,12 @@ class Pages extends StatelessWidget {
   (Size, Size?) calcPageSizesScaled(Size parentSize) {
     if (config.twoPageMode) {
       Size leftPageSizeUnscaled = _getPageSizeUnscaled(currentPageNumber);
-      Size rightPageSizeUnscaled = _getPageSizeUnscaled(currentPageNumber + 1);
+      Size rightPageSizeUnscaled;
+      if (numPages > currentPageNumber) {
+        rightPageSizeUnscaled = _getPageSizeUnscaled(currentPageNumber + 1);
+      } else {
+        rightPageSizeUnscaled = leftPageSizeUnscaled;
+      }
       Size combinedPageSizesUnscaled = Size(
         leftPageSizeUnscaled.width + rightPageSizeUnscaled.width,
         max(leftPageSizeUnscaled.height, rightPageSizeUnscaled.height),
@@ -389,7 +394,7 @@ class Pages extends StatelessWidget {
   }
 
   Size _getPageSizeUnscaled(int pageNumber) {
-    return document.pages.elementAt(pageNumber).size;
+    return document.pages.elementAt(pageNumber - 1).size;
   }
 
   Size _calcScaledPageSize(Size parentSize, Size pageSize) {