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;
};