diff --git a/CHANGELOG.md b/CHANGELOG.md
index 155cb7a..f616304 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@
 ### Changed
 
 * Healthcheck-based dependencies in `service.depends_on`.
+* The `project.name` option is now mandatory.
 
 ### Added
 
diff --git a/docs/modules/ROOT/partials/NixOSOptions.adoc b/docs/modules/ROOT/partials/NixOSOptions.adoc
index 690b23b..eb34a3d 100644
--- a/docs/modules/ROOT/partials/NixOSOptions.adoc
+++ b/docs/modules/ROOT/partials/NixOSOptions.adoc
@@ -112,17 +112,14 @@ Name of the project.
 
 See link:https://docs.docker.com/compose/reference/envvars/#compose_project_name[COMPOSE_PROJECT_NAME]
 
+This is not optional, because getting the project name from a directory name tends to produce different results for different repo checkout location names.
+
 
 [discrete]
 === details
 
-Type:: null or string
-Default::
-+
-----
-null
-----
-
+Type:: string
+No Default:: {blank}
 
 No Example:: {blank}
 
diff --git a/examples/full-nixos/arion-compose.nix b/examples/full-nixos/arion-compose.nix
index 1524e3a..94890db 100644
--- a/examples/full-nixos/arion-compose.nix
+++ b/examples/full-nixos/arion-compose.nix
@@ -1,4 +1,5 @@
 {
+  project.name = "full-nixos";
   services.webserver = { pkgs, lib, ... }: {
     nixos.useSystemd = true;
     nixos.configuration.boot.tmpOnTmpfs = true;
diff --git a/examples/nixos-unit/arion-compose.nix b/examples/nixos-unit/arion-compose.nix
index 1edd4e5..642a18e 100644
--- a/examples/nixos-unit/arion-compose.nix
+++ b/examples/nixos-unit/arion-compose.nix
@@ -17,6 +17,7 @@
  */
 
 {
+  project.name = "nixos-unit";
   services.webserver = { config, pkgs, ... }: {
 
     nixos.configuration = {config, lib, options, pkgs, ...}: {
diff --git a/examples/traefik/arion-compose.nix b/examples/traefik/arion-compose.nix
index b6c6f7d..c43627f 100644
--- a/examples/traefik/arion-compose.nix
+++ b/examples/traefik/arion-compose.nix
@@ -9,7 +9,7 @@
 
  */
 { lib, pkgs, ... }: {
-
+  config.project.name = "traefik";
   config.services = {
     traefik = {
       image.command = [
diff --git a/src/haskell/testdata/Arion/NixSpec/arion-compose.json b/src/haskell/testdata/Arion/NixSpec/arion-compose.json
index 972db6f..c26d994 100644
--- a/src/haskell/testdata/Arion/NixSpec/arion-compose.json
+++ b/src/haskell/testdata/Arion/NixSpec/arion-compose.json
@@ -37,7 +37,7 @@
             }
         ],
         "project": {
-            "name": null
+            "name": "unit-test-data"
         },
         "serviceInfo": {
             "webserver": {
diff --git a/src/haskell/testdata/Arion/NixSpec/arion-compose.nix b/src/haskell/testdata/Arion/NixSpec/arion-compose.nix
index dac420d..40c5907 100644
--- a/src/haskell/testdata/Arion/NixSpec/arion-compose.nix
+++ b/src/haskell/testdata/Arion/NixSpec/arion-compose.nix
@@ -1,4 +1,5 @@
 {
+  project.name = "unit-test-data";
   services.webserver = { pkgs, ... }: {
     nixos.useSystemd = true;
     nixos.configuration.boot.tmpOnTmpfs = true;
diff --git a/src/nix/modules/composition/composition.nix b/src/nix/modules/composition/composition.nix
index 37d2854..8b53882 100644
--- a/src/nix/modules/composition/composition.nix
+++ b/src/nix/modules/composition/composition.nix
@@ -16,9 +16,10 @@ in
         Name of the project.
 
         See ${link "https://docs.docker.com/compose/reference/envvars/#compose_project_name" "COMPOSE_PROJECT_NAME"}
+
+        This is not optional, because getting the project name from a directory name tends to produce different results for different repo checkout location names.
       '';
-      type = types.nullOr types.str;
-      default = null;
+      type = types.str;
     };
   };
   config = {