diff --git a/src/disk_io_thread.cpp b/src/disk_io_thread.cpp
index 2ce06e51a..dd9554d39 100644
--- a/src/disk_io_thread.cpp
+++ b/src/disk_io_thread.cpp
@@ -38,8 +38,10 @@ POSSIBILITY OF SUCH DAMAGE.
 
 #ifdef _WIN32
 #include <malloc.h>
+#ifndef alloca
 #define alloca(s) _alloca(s)
 #endif
+#endif
 
 #ifdef TORRENT_DISK_STATS
 #include "libtorrent/time.hpp"