diff --git a/src/StoreClient.h b/src/StoreClient.h
index 942f9fc..da97e21 100644
--- a/src/StoreClient.h
+++ b/src/StoreClient.h
@@ -163,12 +163,12 @@ private:
 public:
 
     struct Callback {
-        Callback ():callback_handler(NULL), callback_data(NULL) {}
+        Callback() = default;
 
         Callback (STCB *, void *);
         bool pending() const;
-        STCB *callback_handler;
-        void *callback_data;
+        STCB *callback_handler = nullptr; ///< where to deliver the answer
+        CallbackData cbData; ///< the first STCB callback parameter
         AsyncCall::Pointer notifier;
     } _callback;
 };
diff --git a/src/store_client.cc b/src/store_client.cc
index b09f78a..497ea50 100644
--- a/src/store_client.cc
+++ b/src/store_client.cc
@@ -127,14 +127,14 @@ store_client::finishCallback()
     ++answers;
 
     STCB *temphandler = _callback.callback_handler;
-    void *cbdata = _callback.callback_data;
-    _callback = Callback(nullptr, nullptr);
+    const auto cbdata = _callback.cbData.validDone();
+    _callback.~Callback();
+    new (&_callback) Callback();
     copyInto.data = nullptr;
 
-    if (cbdataReferenceValid(cbdata))
+    if (cbdata)
         temphandler(cbdata, result);
 
-    cbdataReferenceDone(cbdata);
 }
 
 store_client::store_client(StoreEntry *e) :
@@ -194,7 +194,8 @@ store_client::copy(StoreEntry * anEntry,
 #endif
 
     assert(!_callback.pending());
-    _callback = Callback (callback_fn, cbdataReference(data));
+    _callback.~Callback();
+    new (&_callback) Callback(callback_fn, data);
     copyInto.data = copyRequest.data;
     copyInto.length = copyRequest.length;
     copyInto.offset = copyRequest.offset;
@@ -1050,7 +1051,7 @@ store_client::dumpStats(MemBuf * output, int clientNumber) const
     if (_callback.pending())
         return;
 
-    output->appendf("\tClient #%d, %p\n", clientNumber, _callback.callback_data);
+    output->appendf("\tClient #%d, %p\n", clientNumber, this);
     output->appendf("\t\tcopy_offset: %" PRId64 "\n", copyInto.offset);
     output->appendf("\t\tcopy_size: %" PRIuSIZE "\n", copyInto.length);
     output->append("\t\tflags:", 8);
@@ -1073,7 +1074,7 @@ store_client::Callback::pending() const
     return callback_handler && !notifier;
 }
 
-store_client::Callback::Callback(STCB *function, void *data) : callback_handler(function), callback_data (data) {}
+store_client::Callback::Callback(STCB *function, void *data) : callback_handler(function), cbData (data) {}
 
 #if USE_DELAY_POOLS
 int