diff --git a/library/aead_chacha20_poly1305.c b/library/aead_chacha20_poly1305.c
index 3aa8d63..dac96ae 100644
--- a/library/aead_chacha20_poly1305.c
+++ b/library/aead_chacha20_poly1305.c
@@ -430,7 +430,7 @@
     {
         if ( verbose != 0 )
         {
-            mbedtls_printf( "  ChaCha20-Poly1305 test %zi ", i );
+            mbedtls_printf( "  ChaCha20-Poly1305 test %zu ", i );
         }
 
         result = mbedtls_aead_chacha20_poly1305_crypt_and_mac( test_key[i],