diff --git a/src/libstore/filetransfer.cc b/src/libstore/filetransfer.cc
index f2430631d..1525ef5fb 100644
--- a/src/libstore/filetransfer.cc
+++ b/src/libstore/filetransfer.cc
@@ -261,7 +261,7 @@ struct curlFileTransfer : public FileTransfer
             return ((TransferItem *) userp)->headerCallback(contents, size, nmemb);
         }
 
-        int progressCallback(double dltotal, double dlnow)
+        int progressCallback(curl_off_t dltotal, curl_off_t dlnow)
         {
             try {
                 act.progress(dlnow, dltotal);
@@ -271,17 +271,17 @@ struct curlFileTransfer : public FileTransfer
             return getInterrupted();
         }
 
-        int silentProgressCallback(double dltotal, double dlnow)
+        int silentProgressCallback(curl_off_t dltotal, curl_off_t dlnow)
         {
             return getInterrupted();
         }
 
-        static int progressCallbackWrapper(void * userp, double dltotal, double dlnow, double ultotal, double ulnow)
+        static int progressCallbackWrapper(void * userp, curl_off_t dltotal, curl_off_t dlnow, curl_off_t ultotal, curl_off_t ulnow)
         {
             return ((TransferItem *) userp)->progressCallback(dltotal, dlnow);
         }
 
-        static int silentProgressCallbackWrapper(void * userp, double dltotal, double dlnow, double ultotal, double ulnow)
+        static int silentProgressCallbackWrapper(void * userp, curl_off_t dltotal, curl_off_t dlnow, curl_off_t ultotal, curl_off_t ulnow)
         {
             return ((TransferItem *) userp)->silentProgressCallback(dltotal, dlnow);
         }