Please note: DO NOT use non TS (thermal sensor SPD chip) equipped modules.
If you do, the fan will run at full speed and this can be very bad for the machine not least irritating.
I looked into installing aftermarket 4GBs in my 2011 iMac but ran into the exact same problem.
Had to put existing memory back in, but its feasible to swap the SPD chip(s) and then simply copy across the data altering the checksum to ignore the variable byte.
Typically it has to be done before soldering, then verified to make sure nothing has changed.
Incidentally its also possible to use these modules in a conventional laptop and having the thermal sensor can make things far more reliable as typically 2GB modules run quite hot at the best of times.