diff --git a/library/bignum_core.c b/library/bignum_core.c
index dfed60d55b..f66739df92 100644
--- a/library/bignum_core.c
+++ b/library/bignum_core.c
@@ -856,8 +856,8 @@ mbedtls_mpi_uint mbedtls_mpi_core_sub_int(mbedtls_mpi_uint *X,
     return c;
 }
 
-mbedtls_mpi_uint mbedtls_mpi_core_check_zero_ct(const mbedtls_mpi_uint *A,
-                                                size_t limbs)
+mbedtls_ct_condition_t mbedtls_mpi_core_check_zero_ct(const mbedtls_mpi_uint *A,
+                                                      size_t limbs)
 {
     mbedtls_mpi_uint bits = 0;
 
@@ -865,7 +865,7 @@ mbedtls_mpi_uint mbedtls_mpi_core_check_zero_ct(const mbedtls_mpi_uint *A,
         bits |= A[i];
     }
 
-    return bits;
+    return mbedtls_ct_bool(bits);
 }
 
 void mbedtls_mpi_core_to_mont_rep(mbedtls_mpi_uint *X,
diff --git a/library/bignum_core.h b/library/bignum_core.h
index b56be0a714..92c8d47db5 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -662,11 +662,11 @@ mbedtls_mpi_uint mbedtls_mpi_core_sub_int(mbedtls_mpi_uint *X,
  * \param[in] A   The MPI to test.
  * \param limbs   Number of limbs in \p A.
  *
- * \return        0 if `A == 0`
- *                non-0 (may be any value) if `A != 0`.
+ * \return        MBEDTLS_CT_FALSE if `A == 0`
+ *                MBEDTLS_CT_TRUE  if `A != 0`.
  */
-mbedtls_mpi_uint mbedtls_mpi_core_check_zero_ct(const mbedtls_mpi_uint *A,
-                                                size_t limbs);
+mbedtls_ct_condition_t mbedtls_mpi_core_check_zero_ct(const mbedtls_mpi_uint *A,
+                                                      size_t limbs);
 
 /**
  * \brief          Returns the number of limbs of working memory required for