address comments from Davide

Change-Id: I4450a345df616266270e0884c4a7c49994ebf54f
diff --git a/tests/unit-tests/crypto-helpers.t.cpp b/tests/unit-tests/crypto-helpers.t.cpp
index febd0be..5834382 100644
--- a/tests/unit-tests/crypto-helpers.t.cpp
+++ b/tests/unit-tests/crypto-helpers.t.cpp
@@ -312,8 +312,9 @@
                                         (uint8_t*)associatedData.c_str(), associatedData.size(), counter);
   auto decoded = decodeBlockWithAesGcm128(block, key, (uint8_t*)associatedData.c_str(), associatedData.size());
   BOOST_CHECK_EQUAL(plaintext, std::string(decoded.get<char>(), decoded.size()));
-  decoded = decodeBlockWithAesGcm128(block, key, (uint8_t*)wrongAssociatedData.c_str(), wrongAssociatedData.size());
-  BOOST_CHECK_EQUAL(decoded.size(), 0);
+  BOOST_CHECK_THROW(decodeBlockWithAesGcm128(block, key,
+                                             (uint8_t*)wrongAssociatedData.c_str(),
+                                             wrongAssociatedData.size()), std::runtime_error);
 }
 
 BOOST_AUTO_TEST_SUITE_END()