diff --git a/library/bignum.c b/library/bignum.c
index dfe976d..0d363e6 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2334,7 +2334,7 @@
     MBEDTLS_MPI_CHK( mbedtls_mpi_lset( X, 0 ) );
 
     Xp = (unsigned char*) X->p;
-    f_rng( p_rng, Xp + overhead, size );
+    MBEDTLS_MPI_CHK( f_rng( p_rng, Xp + overhead, size ) );
 
     mpi_bigendian_to_host( X->p, limbs );