diff --git a/library/bignum.c b/library/bignum.c
index a0964e348..0aee14924 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -198,7 +198,7 @@ int mbedtls_mpi_copy( mbedtls_mpi *X, const mbedtls_mpi *Y )
     if( X == Y )
         return( 0 );
 
-    if( Y->p == NULL )
+    if( Y->n == 0 )
     {
         mbedtls_mpi_free( X );
         return( 0 );