diff --git a/library/bignum_core.c b/library/bignum_core.c
index 7a77f0720..308f5c76f 100644
--- a/library/bignum_core.c
+++ b/library/bignum_core.c
@@ -458,11 +458,10 @@ mbedtls_mpi_uint mbedtls_mpi_core_add_if(mbedtls_mpi_uint *X,
 {
     mbedtls_mpi_uint c = 0;
 
-    /* all-bits 0 if cond is 0, all-bits 1 if cond is non-0 */
-    const mbedtls_mpi_uint mask = mbedtls_ct_mpi_uint_mask(cond);
+    mbedtls_ct_condition_t do_add = mbedtls_ct_bool(cond);
 
     for (size_t i = 0; i < limbs; i++) {
-        mbedtls_mpi_uint add = mask & A[i];
+        mbedtls_mpi_uint add = mbedtls_ct_mpi_uint_if0(do_add, A[i]);
         mbedtls_mpi_uint t = c + X[i];
         c = (t < X[i]);
         t += add;