mixed UDIMM RDIMM memory error when all memory is ECC Registered

We have an R620 which has 16 x 16GB. They were supplied originally by Dell and have the part number M393B2G70DB0 printed on them and so a quick google reveals this spec
www.memory4less.com/m4l_itemdetail.aspx
i.e. RDIMMS which are ECC Registered and Dual Rank

We purchased 8 x 32GB RDIMMS from Dell which have the part number M386B4G70DM0 on them so again a quick google reveals this spec
www.memory4less.com/m4l_itemdetail.aspx
So they are also ECC Registered and Quad Rank

Having read the tech spec I presumed that I would be able to put
8 x 32GB in A1,2,3,4 and B1,2,3,4
8 x 16GB in A5,6,7,8 and B 5,6,7,8

In particular I noted
- Quad rank has to go in 1,2,3,4 (White slots)
- Dual Rank should go in 5,6,7,8 (Black slots)
- Green slots are left blank

However when I did that I got an error on boot up before even the BIOS was loaded than complained about mixed UDIMM and RDIMM and saying to check memory in A5.

Have I missed something?