diff --git a/tests/conv.cxx b/tests/conv.cxx
index 2ee523a..840f18d 100644
--- a/tests/conv.cxx
+++ b/tests/conv.cxx
@@ -266,7 +266,7 @@
 static void doTests(const rfb::PixelFormat &dstpf,
                     const rfb::PixelFormat &srcpf)
 {
-  int i;
+  size_t i;
   char dstb[256], srcb[256];
 
   dstpf.print(dstb, sizeof(dstb));