diff --git a/tests/src/bignum_helpers.c b/tests/src/bignum_helpers.c
index 913f5e387..f6d9f4dea 100644
--- a/tests/src/bignum_helpers.c
+++ b/tests/src/bignum_helpers.c
@@ -71,6 +71,7 @@ int mbedtls_test_read_mpi_core(mbedtls_mpi_uint **pX, size_t *plimbs,
 
 exit:
     mbedtls_free(*pX);
+    *pX = NULL;
     return MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
 }