diff --git a/src/book.cpp b/src/book.cpp
index 6a173ba9c..6d41a87eb 100644
--- a/src/book.cpp
+++ b/src/book.cpp
@@ -87,7 +87,7 @@ void Book::update(const kiwix::Reader& reader)
   m_origId = reader.getOrigId();
   m_articleCount = reader.getArticleCount();
   m_mediaCount = reader.getMediaCount();
-  m_size = reader.getFileSize() << 10;
+  m_size = static_cast<uint64_t>(reader.getFileSize()) << 10;
 
   reader.getFavicon(m_favicon, m_faviconMimeType);
 }
diff --git a/src/reader.cpp b/src/reader.cpp
index 00ac9a48e..abe704de6 100644
--- a/src/reader.cpp
+++ b/src/reader.cpp
@@ -851,7 +851,7 @@ bool Reader::isCorrupted() const
 unsigned int Reader::getFileSize() const
 {
   zim::File* file = this->getZimFileHandler();
-  zim::offset_type size = 0;
+  zim::size_type size = 0;
 
   if (file != NULL) {
     size = file->getFilesize();