aurman: Installation fails when install file doesn't exist

After a successful build, the package fails to install. I assume this happens only when the package doesn’t have a .install file.

$ python main.py -Su
:: Starting full system upgrade...
 there is nothing to do
AUR Packages: nlohmann-json auracle-git
Repo Packages: 
Do you want to install these packages? Y/n: y
Do you want to view the changes of PKGBUILD of nlohmann-json ? N/y: y
Do you want to edit PKGBUILD? N/y: y
Do you want to use the files as they are now? Y/n: y
==> Making package: nlohmann-json 3.1.0-1 (Thu  8 Feb 10:31:52 CET 2018)
....
==> Cleaning up...
2018-02-08 10:38:59,298 - package_classes - install - ERROR - install file of nlohmann-json not available
2018-02-08 10:38:59,298 - main - <module> - ERROR - Unknown exception occurred.
Traceback (most recent call last):
  File "main.py", line 167, in <module>
    process(argv[1:])
  File "main.py", line 162, in process
    package.install(args_with_asdeps)
  File "/home/remy/code/aurman/src/package_classes.py", line 134, in install
    raise InvalidInput()
own_exceptions.InvalidInput