diff --git a/src/encauth/ccm/ccm_memory.c b/src/encauth/ccm/ccm_memory.c
index 3e1920a..de84281 100644
--- a/src/encauth/ccm/ccm_memory.c
+++ b/src/encauth/ccm/ccm_memory.c
@@ -378,7 +378,9 @@ int ccm_memory(int cipher,
    }
 
 #ifdef LTC_CLEAN_STACK
+#ifdef LTC_FAST
    fastMask = 0;
+#endif
    mask = 0;
    zeromem(skey,   sizeof(*skey));
    zeromem(PAD,    sizeof(PAD));