diff --git a/library/bignum.c b/library/bignum.c
index 8848913fb5..7c41b5591a 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1767,9 +1767,15 @@ 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
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     size_t window_bitsize;