diff --git a/library/bignum.c b/library/bignum.c
index eecdf1e99c..c45fd5bf24 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1692,10 +1692,12 @@ int mbedtls_mpi_exp_mod(mbedtls_mpi *X, const mbedtls_mpi *A,
     /*
      * Convert to and from Montgomery around mbedtls_mpi_core_exp_mod().
      */
-    mbedtls_mpi_uint mm = mbedtls_mpi_core_montmul_init(N->p);
-    mbedtls_mpi_core_to_mont_rep(X->p, X->p, N->p, N->n, mm, RR.p, T);
-    mbedtls_mpi_core_exp_mod(X->p, X->p, N->p, N->n, E->p, E->n, RR.p, T);
-    mbedtls_mpi_core_from_mont_rep(X->p, X->p, N->p, N->n, mm, T);
+    {
+        mbedtls_mpi_uint mm = mbedtls_mpi_core_montmul_init(N->p);
+        mbedtls_mpi_core_to_mont_rep(X->p, X->p, N->p, N->n, mm, RR.p, T);
+        mbedtls_mpi_core_exp_mod(X->p, X->p, N->p, N->n, E->p, E->n, RR.p, T);
+        mbedtls_mpi_core_from_mont_rep(X->p, X->p, N->p, N->n, mm, T);
+    }
 
     /*
      * Correct for negative A.