diff --git a/inst/include/testthat/testthat.h b/inst/include/testthat/testthat.h
index de0b5df13..30067b529 100644
--- a/inst/include/testthat/testthat.h
+++ b/inst/include/testthat/testthat.h
@@ -89,7 +89,7 @@ class r_streambuf : public std::streambuf {
     if (n == 1)
       Rprintf("%c", *s);
     else
-      Rprintf("%.*s", n, s);
+      Rprintf("%.*s", static_cast<int>(n), s);
 
     return n;