diff --git a/frontend/document/tilecacheitem.lua b/frontend/document/tilecacheitem.lua
index dd8636f4f..7246fe6fa 100644
--- a/frontend/document/tilecacheitem.lua
+++ b/frontend/document/tilecacheitem.lua
@@ -15,7 +15,7 @@ end
 function TileCacheItem:dump(filename)
     logger.dbg("dumping tile cache to", filename, self.excerpt)
     return serial.dump(self.size, self.excerpt, self.pageno,
-            self.bb.w, self.bb.h, self.bb.stride, self.bb:getType(),
+            self.bb.w, self.bb.h, tonumber(self.bb.stride), self.bb:getType(),
             Blitbuffer.tostring(self.bb), filename)
 end
 
diff --git a/frontend/ui/widget/imagewidget.lua b/frontend/ui/widget/imagewidget.lua
index d861bf12a..7da41350a 100644
--- a/frontend/ui/widget/imagewidget.lua
+++ b/frontend/ui/widget/imagewidget.lua
@@ -205,7 +205,7 @@ function ImageWidget:_loadfile()
                 -- cache this image
                 logger.dbg("cache", hash)
                 cache = ImageCacheItem:new{ bb = self._bb }
-                cache.size = cache.bb.stride * cache.bb.h
+                cache.size = tonumber(cache.bb.stride) * cache.bb.h
                 ImageCache:insert(hash, cache)
             end
         end
diff --git a/plugins/coverbrowser.koplugin/bookinfomanager.lua b/plugins/coverbrowser.koplugin/bookinfomanager.lua
index 4e2180244..ec4a377bb 100644
--- a/plugins/coverbrowser.koplugin/bookinfomanager.lua
+++ b/plugins/coverbrowser.koplugin/bookinfomanager.lua
@@ -450,7 +450,7 @@ function BookInfoManager:extractBookInfo(filepath, cover_specs)
                     dbrow.cover_w = cbb_w
                     dbrow.cover_h = cbb_h
                     dbrow.cover_btype = cover_bb:getType()
-                    dbrow.cover_bpitch = cover_bb.stride
+                    dbrow.cover_bpitch = tonumber(cover_bb.stride)
                     local cover_data = Blitbuffer.tostring(cover_bb)
                     cover_bb:free() -- free bb before compressing to save memory
                     dbrow.cover_datalen = cover_data:len()