diff --git a/artifacts/nickel/test.ncl b/artifacts/nickel/test.ncl
index 2050840..31a40cd 100644
--- a/artifacts/nickel/test.ncl
+++ b/artifacts/nickel/test.ncl
@@ -3,7 +3,6 @@
   artifact_url = "http://localhost.com/artifact.zip",
   type = "zip",
   doi = "...",
-  comment = "",
   virtualization = "docker",
   buildfile_dir = "./",
   package_managers = [ "dpkg", "pip" ],
@@ -12,6 +11,5 @@
   ],
   misc_packages = [
     { name = "mpkg1", url = "http://localhost.com/package1.zip" }
-  ],
-  python_venvs = []
+  ]
 }
diff --git a/workflow/nickel/artifact_contract.ncl b/workflow/nickel/artifact_contract.ncl
index bc691d4..247db90 100644
--- a/workflow/nickel/artifact_contract.ncl
+++ b/workflow/nickel/artifact_contract.ncl
@@ -52,7 +52,9 @@ in
   Artifact = {
     version
       | doc "Version of the Nickel contract"
-      | String,
+      | optional
+      | String
+      | default = "1.0",
     artifact_url
       | doc "URL where to download the artifact"
       | URL,
@@ -64,7 +66,9 @@ in
       | String,
     comment
       | doc "Additional comments on the artifact that couldn't be written elsewhere"
-      | String,
+      | optional
+      | String
+      | default = "",
     virtualization
       | doc "Virtualization technology used in the artifact. Possible values: docker"
       | VirtTech,
@@ -73,15 +77,23 @@ in
       | FilePath,
     package_managers
       | doc "Package Managers used in the container. Possible values: dpkg, rpm, pacman, pip, conda"
-      | Array PackageManager,
+      | optional
+      | Array PackageManager
+      | default = [],
     git_packages
       | doc "Git repositories cloned in the container"
-      | Array GitPackage,
+      | optional
+      | Array GitPackage
+      | default = [],
     misc_packages
       | doc "Misc. packages downloaded from the container"
-      | Array MiscPackage,
+      | optional
+      | Array MiscPackage
+      | default = [],
     python_venvs
       | doc "Python venvs created in the container"
+      | optional
       | Array PythonVenv
+      | default = [],
   }
 }