Test that WithDataFS refuses a nil module and a manifest's reads refuses a repeated module

shipped_data_test.go

@@ -1,6 +1,7 @@
 package fejkdata
 
 import (
+	"io/fs"
 	"reflect"
 	"slices"
 	"strings"
@@ -134,6 +135,13 @@ func TestWithDataFSLayersEachModuleOverTheOneBefore(t *testing.T) {
 	}
 }
 
+func TestWithDataFSRefusesANilModule(t *testing.T) {
+	var unset fs.FS
+	if _, err := New(WithDataFS(fstest.MapFS{}, unset)); err == nil || !strings.Contains(err.Error(), "WithDataFS module 2 is nil") {
+		t.Errorf("New(WithDataFS(fsys, nil)) = %v, want module 2 named nil", err)
+	}
+}
+
 func TestFakeIsSafeForConcurrentUse(t *testing.T) {
 	f, err := New(withShipped(), WithSeed(1))
 	if err != nil {