diff --git a/tests/src/helpers.c b/tests/src/helpers.c
index 8f4d7f2..4f976a2 100644
--- a/tests/src/helpers.c
+++ b/tests/src/helpers.c
@@ -332,7 +332,7 @@
 #endif /* MBEDTLS_TEST_HOOKS */
 
 #if defined(MBEDTLS_BIGNUM_C)
-int mbedtls_test_read_mpi( mbedtls_mpi *X, int radix, const char *s )
+int mbedtls_test_read_mpi( mbedtls_mpi *X, const char *s )
 {
     /* mbedtls_mpi_read_string() currently retains leading zeros.
      * It always allocates at least one limb for the value 0. */
@@ -342,6 +342,6 @@
         return( 0 );
     }
     else
-        return( mbedtls_mpi_read_string( X, radix, s ) );
+        return( mbedtls_mpi_read_string( X, 16, s ) );
 }
 #endif