diff --git a/programs/pkey/pk_sign.c b/programs/pkey/pk_sign.c
index a354e5b..4e755c0 100644
--- a/programs/pkey/pk_sign.c
+++ b/programs/pkey/pk_sign.c
@@ -96,7 +96,7 @@
                                (const unsigned char *) pers,
                                strlen( pers ) ) ) != 0 )
     {
-        mbedtls_printf( " failed\n  ! mbedtls_ctr_drbg_seed returned -0x%04x\n", -ret );
+        mbedtls_printf( " failed\n  ! mbedtls_ctr_drbg_seed returned -0x%04x\n", (unsigned int) -ret );
         goto exit;
     }
 
@@ -127,7 +127,7 @@
     if( ( ret = mbedtls_pk_sign( &pk, MBEDTLS_MD_SHA256, hash, 0, buf, &olen,
                          mbedtls_ctr_drbg_random, &ctr_drbg ) ) != 0 )
     {
-        mbedtls_printf( " failed\n  ! mbedtls_pk_sign returned -0x%04x\n", -ret );
+        mbedtls_printf( " failed\n  ! mbedtls_pk_sign returned -0x%04x\n", (unsigned int) -ret );
         goto exit;
     }