function! SaveUnnamedRegTo()
  call inputsave()
  let regKey = input('Enter key: ')
  call inputrestore()
  let "@" . regKey = @@
endfunction

"When I call the function and feed it a letter:
Enter key: hError detected while processing function SaveUnnamedRegTo:
line 4:
E475: Invalid argument: "@" . regKey = @@
Press ENTER or type command to continue