diff --git a/library/bignum.c b/library/bignum.c
index 38df1addb6..8cf52f4030 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1736,9 +1736,15 @@ cleanup:
 /*
 * Sliding-window exponentiation: X = A^E mod N  (HAC 14.85)
 */
+#if !defined(MBEDTLS_MPI_EXP_MOD_ALT_FALLBACK)
 int mbedtls_mpi_exp_mod(mbedtls_mpi *X, const mbedtls_mpi *A,
                         const mbedtls_mpi *E, const mbedtls_mpi *N,
                         mbedtls_mpi *prec_RR)
+#else
+int mbedtls_mpi_exp_mod_soft(mbedtls_mpi *X, const mbedtls_mpi *A,
+                             const mbedtls_mpi *E, const mbedtls_mpi *N,
+                             mbedtls_mpi *prec_RR)
+#endif
 {
     return mbedtls_mpi_exp_mod_optionally_safe(X, A, E, MBEDTLS_MPI_IS_SECRET, N, prec_RR);
 }