diff --git a/common/rfb/util.cxx b/common/rfb/util.cxx
index 22e00ff..cfec2ef 100644
--- a/common/rfb/util.cxx
+++ b/common/rfb/util.cxx
@@ -44,7 +44,7 @@
 
   void CharArray::format(const char *fmt, ...) {
     va_list ap;
-    size_t len;
+    int len;
 
     va_start(ap, fmt);
     len = vsnprintf(NULL, 0, fmt, ap);