diff --git a/library/cipher.c b/library/cipher.c
index 485a09b..0e1224c 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -245,7 +245,7 @@
     if( NULL == cipher_info || NULL == ctx )
         return POLARSSL_ERR_CIPHER_BAD_INPUT_DATA;
 
-    memset( ctx, 0, sizeof( ctx ) );
+    memset( ctx, 0, sizeof( cipher_context_t ) );
 
     if( NULL == ( ctx->cipher_ctx = cipher_info->base->ctx_alloc_func() ) )
         return POLARSSL_ERR_CIPHER_ALLOC_FAILED;