diff --git a/library/poly1305.c b/library/poly1305.c
index d7c9ce1..004d857 100644
--- a/library/poly1305.c
+++ b/library/poly1305.c
@@ -492,7 +492,7 @@
     {
         if ( verbose != 0 )
         {
-            mbedtls_printf( "  Poly1305 test %zi ", i );
+            mbedtls_printf( "  Poly1305 test %zu ", i );
         }
 
         result = mbedtls_poly1305_mac( test_keys[i],