diff --git a/programs/test/udp_proxy.c b/programs/test/udp_proxy.c
index 7447571..d601dcc 100644
--- a/programs/test/udp_proxy.c
+++ b/programs/test/udp_proxy.c
@@ -456,7 +456,7 @@
     {
         if( ( ret = ctx_buffer_flush( buf ) ) <= 0 )
         {
-            mbedtls_printf( "ctx_buffer_flush failed with -%#04x", -ret );
+            mbedtls_printf( "ctx_buffer_flush failed with -%#04x", (unsigned int) -ret );
             return( ret );
         }
     }
@@ -495,7 +495,7 @@
     ret = mbedtls_net_send( ctx, data, len );
     if( ret < 0 )
     {
-        mbedtls_printf( "net_send returned -%#04x\n", -ret );
+        mbedtls_printf( "net_send returned -%#04x\n", (unsigned int) -ret );
     }
     return( ret );
 }
@@ -999,7 +999,7 @@
     {
         char error_buf[100];
         mbedtls_strerror( ret, error_buf, 100 );
-        mbedtls_printf( "Last error was: -0x%04X - %s\n\n", - ret, error_buf );
+        mbedtls_printf( "Last error was: -0x%04X - %s\n\n", (unsigned int) -ret, error_buf );
         fflush( stdout );
     }
 #endif