diff --git a/src/bigtiffimage.cpp b/src/bigtiffimage.cpp
index 4b5e3239..0d1c7b22 100644
--- a/src/bigtiffimage.cpp
+++ b/src/bigtiffimage.cpp
@@ -244,7 +244,7 @@ namespace Exiv2
                                 throw Error(57);
                             }
 
-                            DataBuf buf(allocate);
+                            DataBuf buf(static_cast<long>(allocate));
 
                             const uint64_t offset = header_.format() == Header::StandardTiff?
                                     byteSwap4(data, 0, doSwap_):