diff --git a/test/model/extensions/classes/first.yml b/test/model/extensions/classes/first.yml
new file mode 100644
index 0000000..96ece27
--- /dev/null
+++ b/test/model/extensions/classes/first.yml
@@ -0,0 +1,6 @@
+parameters:
+  _param:
+    some: param
+  lab:
+    name: test
+    label: first
diff --git a/test/model/extensions/classes/lab/env/dev.yml b/test/model/extensions/classes/lab/env/dev.yml
new file mode 100644
index 0000000..0cce363
--- /dev/null
+++ b/test/model/extensions/classes/lab/env/dev.yml
@@ -0,0 +1,4 @@
+
+parameters:
+  lab:
+    name: dev
diff --git a/test/model/extensions/classes/second.yml b/test/model/extensions/classes/second.yml
new file mode 100644
index 0000000..a9babd3
--- /dev/null
+++ b/test/model/extensions/classes/second.yml
@@ -0,0 +1,8 @@
+classes:
+- first
+
+parameters:
+  will:
+    warn:
+      at:
+        second: ${_param:notfound}
diff --git a/test/model/extensions/classes/third.yml b/test/model/extensions/classes/third.yml
new file mode 100644
index 0000000..20a937c
--- /dev/null
+++ b/test/model/extensions/classes/third.yml
@@ -0,0 +1,13 @@
+classes:
+- missing.class
+- second
+
+parameters:
+  _param:
+    notfound: exist
+  myparam: ${_param:some}
+  will:
+    not:
+      fail:
+        at:
+          tree: ${_param:notfound}