diff --git a/tests/include/test/helpers.h b/tests/include/test/helpers.h
index ab9a0c3..c0d2c00 100644
--- a/tests/include/test/helpers.h
+++ b/tests/include/test/helpers.h
@@ -59,6 +59,10 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#if defined(MBEDTLS_BIGNUM_C)
+#include "mbedtls/bignum.h"
+#endif
+
 typedef enum
 {
     MBEDTLS_TEST_RESULT_SUCCESS = 0,
@@ -210,4 +214,25 @@
                                  const char *file, int line);
 #endif
 
+#if defined(MBEDTLS_BIGNUM_C)
+/** Read an MPI from a string.
+ *
+ * Like mbedtls_mpi_read_string(), but size the resulting bignum based
+ * on the number of digits in the string. In particular, construct 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 radix         The radix (2 to 16).
+ * \param[in] s         The null-terminated string to read from.
+ *
+ * \return \c 0 on success, an \c MBEDTLS_ERR_MPI_xxx error code otherwise.
+ */
+/* Since the library has exactly the desired behavior, this is trivial. */
+int mbedtls_test_read_mpi( mbedtls_mpi *X, int radix, const char *s );
+#endif /* MBEDTLS_BIGNUM_C */
+
 #endif /* TEST_HELPERS_H */