diff --git a/library/bignum.c b/library/bignum.c
index 768198229..08b8b3429 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1689,7 +1689,9 @@ int mbedtls_mpi_exp_mod(mbedtls_mpi *X, const mbedtls_mpi *A,
      * Correct for negative A.
      */
     if (A->s == -1 && (E->p[0] & 1) != 0) {
-        X->s = -1;
+        mbedtls_ct_condition_t is_x_non_zero = mbedtls_mpi_core_check_zero_ct(X->p, X->n);
+        X->s = mbedtls_ct_uint_if(is_x_non_zero, -1, 1);
+
         MBEDTLS_MPI_CHK(mbedtls_mpi_add_mpi(X, N, X));
     }
 
diff --git a/tests/suites/test_suite_bignum.misc.data b/tests/suites/test_suite_bignum.misc.data
index 8f5218c1f..a953153ab 100644
--- a/tests/suites/test_suite_bignum.misc.data
+++ b/tests/suites/test_suite_bignum.misc.data
@@ -1362,6 +1362,9 @@ mpi_exp_mod:"04":"00":"09":"1":0
 Test mbedtls_mpi_exp_mod: 10 ^ 0 (1 limb) mod 9
 mpi_exp_mod:"0a":"00":"09":"1":0
 
+Test mbedtls_mpi_exp_mod: -3 ^ 3 mod 27
+mpi_exp_mod:"-3":"3":"1b":"1b":0
+
 Test mbedtls_mpi_exp_mod: MAX_SIZE exponent
 mpi_exp_mod_size:2:MBEDTLS_MPI_MAX_SIZE:10:"":0