diff --git a/library/bignum.c b/library/bignum.c
index d53aefd..d56a16e 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2938,7 +2938,7 @@
 cleanup:
 
     if( ret != 0 && verbose != 0 )
-        mbedtls_printf( "Unexpected error, return code = %08X\n", ret );
+        mbedtls_printf( "Unexpected error, return code = %08X\n", (unsigned int) ret );
 
     mbedtls_mpi_free( &A ); mbedtls_mpi_free( &E ); mbedtls_mpi_free( &N ); mbedtls_mpi_free( &X );
     mbedtls_mpi_free( &Y ); mbedtls_mpi_free( &U ); mbedtls_mpi_free( &V );