diff --git a/tests/include/test/bignum_helpers.h b/tests/include/test/bignum_helpers.h
index 2f6bf89..cf175a3 100644
--- a/tests/include/test/bignum_helpers.h
+++ b/tests/include/test/bignum_helpers.h
@@ -77,30 +77,22 @@
  *
  * - This function guarantees that if \p s begins with '-' then the sign
  *   bit of the result will be negative, even if the value is 0.
- *   When this function encounters such a "negative 0", it
- *   increments #mbedtls_test_case_uses_negative_0.
- * - The size of the result is exactly the minimum number of limbs needed
- *   to fit the digits in the input. In particular, this function constructs
- *   a bignum with 0 limbs for an empty string, and a bignum with leading 0
- *   limbs if the string has sufficiently many leading 0 digits.
- *   This is important so that the "0 (null)" and "0 (1 limb)" and
- *   "leading zeros" test cases do what they claim.
+ *   When this function encounters such a "negative 0", it calls
+ *   mbedtls_test_increment_case_uses_negative_0().
+ * - The size of the result is exactly the minimum number of limbs needed to fit
+ *   the digits in the input. In particular, this function constructs a bignum
+ *   with 0 limbs for an empty string, and a bignum with leading 0 limbs if the
+ *   string has sufficiently many leading 0 digits. This is important so that
+ *   the "0 (null)" and "0 (1 limb)" and "leading zeros" test cases do what they
+ *   claim.
  *
- * \param[out] X        The MPI object to populate. It must be initialized.
- * \param[in] s         The null-terminated hexadecimal string to read from.
+ * \param[out] X The MPI object to populate. It must be initialized.
+ * \param[in] s The null-terminated hexadecimal string to read from.
  *
  * \return \c 0 on success, an \c MBEDTLS_ERR_MPI_xxx error code otherwise.
  */
 int mbedtls_test_read_mpi(mbedtls_mpi *X, const char *s);
 
-/** Nonzero if the current test case had an input parsed with
- * mbedtls_test_read_mpi() that is a negative 0 (`"-"`, `"-0"`, `"-00"`, etc.,
- * constructing a result with the sign bit set to -1 and the value being
- * all-limbs-0, which is not a valid representation in #mbedtls_mpi but is
- * tested for robustness).
- */
-extern unsigned mbedtls_test_case_uses_negative_0;
-
 #endif /* MBEDTLS_BIGNUM_C */
 
 #endif /* TEST_BIGNUM_HELPERS_H */
diff --git a/tests/include/test/helpers.h b/tests/include/test/helpers.h
index 564a553..b672ecc 100644
--- a/tests/include/test/helpers.h
+++ b/tests/include/test/helpers.h
@@ -74,6 +74,9 @@
 #if defined(MBEDTLS_TEST_MUTEX_USAGE)
     const char *mutex_usage_error;
 #endif
+#if defined(MBEDTLS_BIGNUM_C)
+    unsigned case_uses_negative_0;
+#endif
 }
 mbedtls_test_info_t;
 
@@ -151,6 +154,28 @@
 void mbedtls_test_set_mutex_usage_error(const char *msg);
 #endif
 
+#if defined(MBEDTLS_BIGNUM_C)
+
+/**
+ * \brief           Get whether the current test is a bignum test that uses
+ *                  negative zero.
+ *
+ * \return          non zero if the current test uses bignum negative zero.
+ */
+unsigned mbedtls_test_get_case_uses_negative_0(void);
+
+/**
+ * \brief           Indicate that the current test uses bignum negative zero.
+ *
+ * \note            This function is called if the current test case had an
+ *                  input parsed with mbedtls_test_read_mpi() that is a negative
+ *                  0 (`"-"`, `"-0"`, `"-00"`, etc., constructing a result with
+ *                  the sign bit set to -1 and the value being all-limbs-0,
+ *                  which is not a valid representation in #mbedtls_mpi but is
+ *                  tested for robustness). *
+ */
+void  mbedtls_test_increment_case_uses_negative_0(void);
+#endif
 
 int mbedtls_test_platform_setup(void);
 void mbedtls_test_platform_teardown(void);