diff --git a/library/bignum.c b/library/bignum.c
index 5ddcf72..0b8fec3 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1639,6 +1639,7 @@
             *prec_RR = RR;
         }
     } else {
+        MBEDTLS_MPI_CHK(mbedtls_mpi_grow(prec_RR, N->n));
         RR = *prec_RR;
     }