diff --git a/testing/regtest.cc b/testing/regtest.cc
index 12b712e..8219cc0 100644
--- a/testing/regtest.cc
+++ b/testing/regtest.cc
@@ -12,7 +12,7 @@ public:
     Options() : encode_srcwin_maxsz(1<<20), 
 		block_size(Constants::BLOCK_SIZE),
 		size_known(false) { }
-    size_t encode_srcwin_maxsz;
+    uint64_t encode_srcwin_maxsz;
     size_t block_size;
     bool size_known;
   };