diff --git a/Lake/LeanConfig.lean b/Lake/LeanConfig.lean
index bb90519f06..9de4e609ac 100644
--- a/Lake/LeanConfig.lean
+++ b/Lake/LeanConfig.lean
@@ -29,7 +29,8 @@ unsafe def fromLeanFileUnsafe
         env.evalConstCheck PackageConfig Options.empty ``PackageConfig `package
       match configE with
       | Except.ok config => Package.mk root config
-      | Except.error error => throw <| IO.userError error
+      | Except.error error => throw <| IO.userError <|
+        s!"unexpected type at 'package', `Lake.Packager` or `Lake.PackageConfig` expected"
   else
     throw <| IO.userError <| s!"package configuration (at {path}) has errors"
 
diff --git a/examples/io/.gitignore b/examples/io/.gitignore
new file mode 100644
index 0000000000..796b96d1c4
--- /dev/null
+++ b/examples/io/.gitignore
@@ -0,0 +1 @@
+/build
diff --git a/examples/io/Io.lean b/examples/io/Io.lean
new file mode 100644
index 0000000000..00ac27245d
--- /dev/null
+++ b/examples/io/Io.lean
@@ -0,0 +1,2 @@
+def main : IO Unit :=
+  IO.println "Hello from Io!"
diff --git a/examples/io/clean.sh b/examples/io/clean.sh
new file mode 100644
index 0000000000..07cbd15340
--- /dev/null
+++ b/examples/io/clean.sh
@@ -0,0 +1 @@
+rm -r build
diff --git a/examples/io/package.lean b/examples/io/package.lean
new file mode 100644
index 0000000000..58ef709beb
--- /dev/null
+++ b/examples/io/package.lean
@@ -0,0 +1,8 @@
+import Lake.Package
+
+def package : Lake.Packager := fun path args => do
+  IO.println s!"computing io package in {path} with args {args} ..."
+  return {
+    name := "io"
+    version := "1.0"
+  }
diff --git a/examples/io/package.sh b/examples/io/package.sh
new file mode 100644
index 0000000000..b6449c2dd0
--- /dev/null
+++ b/examples/io/package.sh
@@ -0,0 +1 @@
+../../build/bin/Lake build-bin
diff --git a/examples/io/test.sh b/examples/io/test.sh
new file mode 100644
index 0000000000..1872494d40
--- /dev/null
+++ b/examples/io/test.sh
@@ -0,0 +1,3 @@
+./clean.sh
+./package.sh
+./build/bin/io