By inspection of the source code, _ag_account_local_value and _ag_account_global_value appear to expect to be passed a GValue initialized to store a string (G_TYPE_STRING).

However, before reaching the code path that issues this message:

  DEBUG ("no " MC_IDENTITY_KEY " found, synthesising one:");

the GValue has been unset with g_value_unset() (true since commit b29f2a85), which returns it to an all-zeroes state, in which it needs to be re-initialized with g_value_init() to be used.

I believe one correct fix is to insert a call to

  g_value_init (&value, G_TYPE_STRING);

just after that debug message is emitted.

Another possibility would be to change this code added by b29f2a85:

@@ -885,6 +885,10 @@ _ag_accountid_to_mc_key (McdAccountManagerSso *sso,
       g_value_unset (&value);
       return uid;
     }
+  else
+    {
+      g_value_unset (&value);
+    }
 
   if (!create)
     return NULL;

into:

      g_value_unset (&value);
      return uid;
    }
  else if (!create)
    {
      g_value_unset (&value);
      return NULL;
    }