diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 12e9697..4ac8186 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -2405,8 +2405,8 @@
             if ( (size_t)ret > len || ( INT_MAX > SIZE_MAX && ret > SIZE_MAX ) )
             {
                 MBEDTLS_SSL_DEBUG_MSG( 1, 
-                    ( "f_recv returned %d bytes but only %zu were requested", 
-                    ret, len ) );
+                    ( "f_recv returned %d bytes but only %lu were requested", 
+                    ret, (unsigned long)len ) );
                 return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
             }
 
@@ -2460,8 +2460,8 @@
         if( (size_t)ret > ssl->out_left || ( INT_MAX > SIZE_MAX && ret > SIZE_MAX ) )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, 
-                ( "f_send returned %d bytes but only %zu bytes were sent", 
-                ret, ssl->out_left ) );
+                ( "f_send returned %d bytes but only %lu bytes were sent", 
+                ret, (unsigned long)ssl->out_left ) );
             return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
         }