Sort Table command no longer works in Numbers 3.2.2

Question

I am running OS X 10.9.4 and recently updated Numbers to 3.2.2 on my Macbook Pro. Since then I have been unable to sort a table which uses cells referenced from other tables. I have re-created a very simple set of tables to illustrate the problem. Table 1 and 1-1 below are the basic tables created to hold the data. The DATE column is formatted as Date with no Time format.
The Summary Table selects the relevent data from each DATE field in the above 2 tables. I reference these fields by typing in "=" and then clicking on the appropriate field. Shown in Fig. 2.
FIGURE 2.
Everything looks fine in the table until I want to SORT the Summary Table by DATE field which I do by clicking on the down arrow at the top of the DATE column and selecting SORT ASCENDING - see Fig. 3 below.
FIGURE 3.
When I do this, the result is as shown in Fig. 4.
FIGURE 4.
When I click on the exclamation mark the text is as shown in Fig. 5.
Figure 5.
I used to be able to do this Sort really easily before updating to the latest version of Numbers. I have tried creating another worksheet and setting up completely fresh tables, I have created the same tables on my iMac running Numbers 3.2 and I keep getting the same result. Each time I run the Sort command I get varying results of more or less exclamation marks interspersed with blank cells. I may be being completely thick here but I would really appreciate someone helping me out as this is driving me NUTS!! I have searched on as many Forums as I could find and have been unable to find anyone with a similar problem. Any help here would be much appreciated.

Answer 1

Thanks everyone for your replies. I have copied in the screenshots of the tables again by dragging & dropping this time so hopefully you can see them now.
This shows the tables I have created.
This shows the formula in the cell.
The sort command on the column I want to sort on.
The result after the sort command...
And the error message.
Anyway, if you can see all that now, could someone explain what is actually happening in the sort process as I don't really understand why the results appear as they do. I have now taken out all blank rows and columns of data in Table 1 and Table 1-1 and now when I run the sort I get the following result:-
What I want to happen is that the Summary Table is arranged in Date order.  - surely that should be easy enough?? This used to be such a simple process before the update!!
Could someone explain to me what is actually happening here and whether the safe sort suggestion made will work in this instance? I am off to experiment now and have another look at the links given as I still don't really understand what is happening. Thanks again for the suggestions and help - much appreciated!