diff --git a/src/libstore/binary-cache-store.cc b/src/libstore/binary-cache-store.cc
index 7d103e0cc..9682db730 100644
--- a/src/libstore/binary-cache-store.cc
+++ b/src/libstore/binary-cache-store.cc
@@ -283,7 +283,7 @@ void BinaryCacheStore::addToStore(const ValidPathInfo & info, Source & narSource
     if (repair || !fileExists(narInfo->url)) {
         stats.narWrite++;
         upsertFile(narInfo->url,
-            std::make_shared<std::fstream>(fnTemp, std::ios_base::in),
+            std::make_shared<std::fstream>(fnTemp, std::ios_base::in | std::ios_base::binary),
             "application/x-nix-nar");
     } else
         stats.narWriteAverted++;
diff --git a/src/libstore/s3-binary-cache-store.cc b/src/libstore/s3-binary-cache-store.cc
index 1b7dff085..a0a446bd3 100644
--- a/src/libstore/s3-binary-cache-store.cc
+++ b/src/libstore/s3-binary-cache-store.cc
@@ -266,6 +266,10 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
         const std::string & mimeType,
         const std::string & contentEncoding)
     {
+        istream->seekg(0, istream->end);
+        auto size = istream->tellg();
+        istream->seekg(0, istream->beg);
+
         auto maxThreads = std::thread::hardware_concurrency();
 
         static std::shared_ptr<Aws::Utils::Threading::PooledThreadExecutor>
@@ -343,13 +347,11 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
             std::chrono::duration_cast<std::chrono::milliseconds>(now2 - now1)
                 .count();
 
-        auto size = istream->tellg();
-
         printInfo("uploaded 's3://%s/%s' (%d bytes) in %d ms",
             bucketName, path, size, duration);
 
         stats.putTimeMs += duration;
-        stats.putBytes += size;
+        stats.putBytes += std::max(size, (decltype(size)) 0);
         stats.put++;
     }