diff --git a/lib/Epub/Epub/BookMetadataCache.h b/lib/Epub/Epub/BookMetadataCache.h
index 7f45090a..2ff123d2 100644
--- a/lib/Epub/Epub/BookMetadataCache.h
+++ b/lib/Epub/Epub/BookMetadataCache.h
@@ -18,11 +18,11 @@ class BookMetadataCache {
 
   struct SpineEntry {
     std::string href;
-    size_t cumulativeSize;
+    uint32_t cumulativeSize;
     int16_t tocIndex;
 
     SpineEntry() : cumulativeSize(0), tocIndex(-1) {}
-    SpineEntry(std::string href, const size_t cumulativeSize, const int16_t tocIndex)
+    SpineEntry(std::string href, const uint32_t cumulativeSize, const int16_t tocIndex)
         : href(std::move(href)), cumulativeSize(cumulativeSize), tocIndex(tocIndex) {}
   };
 
@@ -44,7 +44,7 @@ class BookMetadataCache {
 
  private:
   std::string cachePath;
-  size_t lutOffset;
+  uint32_t lutOffset;
   uint16_t spineCount;
   uint16_t tocCount;
   bool loaded;