Setting flycheck-clang-include-path in .dir-locals using projectile

Trying to set flycheck-clang-include-path without the need to include the full path of the project include directories using projectile, but I get errors... So this works:

((nil . (
     (company-clang-arguments . (
                     "/home/user/Downloads/project/headers"
                     "/home/user/Downloads/project/source/mon"
                     ))
         (flycheck-clang-include-path . (
                     "/home/user/Downloads/project/headers"
                     "/home/user/Downloads/project/source/mon"
                     ))
     )))

But this does not:

((nil . (
     (company-clang-arguments . (
                     (concat "-I" (projectile-project-root) "headers")
                     (concat "-I" (projectile-project-root) "source/mon")
                     ))
         (flycheck-clang-include-path . (
                     (concat (projectile-project-root) "headers")
                     (concat (projectile-project-root) "source/mon")
                     ))
     )))

The error that is reported:

flycheck-substitute-argument: Value ((concat (projectile-project-root) "headers")) of flycheck-clang-include-path for option "-I" is not a list of strings