diff --git a/tests/suites/helpers.function b/tests/suites/helpers.function
index 71390ec..57bc259 100644
--- a/tests/suites/helpers.function
+++ b/tests/suites/helpers.function
@@ -173,6 +173,33 @@
         memcpy(param_fail_jmp, jmp_tmp, sizeof(jmp_buf));                   \
     } while( 0 )
 
+/**
+ * \brief   This macro tests the statement passed to it as a test step or
+ *          individual test in a test case. The macro assumes the test will not fail.
+ *
+ *          It assumes the library function under test cannot return a value and
+ *          assumes errors can only be indicated by calls to
+ *          MBEDTLS_PARAM_FAILED().
+ *
+ *          When MBEDTLS_CHECK_PARAMS is enabled, calls to the parameter failure
+ *          callback, MBEDTLS_PARAM_FAILED(), are assumed to indicate the
+ *          expected failure. If MBEDTLS_CHECK_PARAMS is not enabled, no test
+ *          can be made.
+ *
+ *          This macro is intended to test that function that return void
+ *          accept all of the parameter values they're supposed to accept - eg
+ *          that they don't call MBEDTLS_PARAM_FAILED() when a parameter
+ *          that's allowed to be NULL happends to be NULL.
+ *
+ *          Note: for functions that return something other that void,
+ *          checking that they accept all the parameters they're supposed to
+ *          accept is best done by using TEST_ASSERT() and checking the return
+ *          value as well.
+ *
+ * \param   TEST                The test expression to be tested.
+ */
+#define TEST_VALID_PARAM( TEST )                                    \
+    TEST_ASSERT( ( TEST, 1 ) );
 #endif /* MBEDTLS_CHECK_PARAMS && !MBEDTLS_PARAM_FAILED_ALT */
 
 #define assert(a) if( !( a ) )                                      \
diff --git a/tests/suites/test_suite_aes.function b/tests/suites/test_suite_aes.function
index 7dab01b..f61f71c 100644
--- a/tests/suites/test_suite_aes.function
+++ b/tests/suites/test_suite_aes.function
@@ -379,6 +379,8 @@
 
     TEST_INVALID_PARAM( mbedtls_aes_init( NULL ) );
 
+    TEST_INVALID_PARAM( mbedtls_aes_xts_init( NULL ) );
+
     /* mbedtls_aes_setkey_enc() */
     TEST_INVALID_PARAM_RET( MBEDTLS_ERR_AES_BAD_INPUT_DATA,
                             mbedtls_aes_setkey_enc( NULL, key, 128 ) );
@@ -393,6 +395,10 @@
     TEST_INVALID_PARAM_RET( MBEDTLS_ERR_AES_BAD_INPUT_DATA,
                             mbedtls_aes_setkey_dec( &dummy_ctx, NULL, 128 ) );
 
+    /* These calls accept NULL */
+    TEST_VALID_PARAM( mbedtls_aes_free( NULL ) );
+    TEST_VALID_PARAM( mbedtls_aes_xts_free( NULL ) );
+
 exit:
     return;
 }