Photos w/aircraft ICAO codes...

I have a question…I’m currently updating my photos that I’ve uploaded to FlightAware and have come across an ICAO code that is displaying as invalid when I try to attach it to an aircraft photo of mine. The aircraft in question is the E-6B Mercury which has a code of E6. Is this just an error in the FlightAware database, or am I wrong with the code I’m trying to enter?

Many thanks!

Thinking it might be listed as a B703, I checked the FAA contractions handbook. It does shoe E6 as the code: faa.gov/air_traffic/publicat … /5-2-E.htm