diff --git a/cmd/pgen/gen.go b/cmd/pgen/gen.go
index b974515..cf83c10 100644
--- a/cmd/pgen/gen.go
+++ b/cmd/pgen/gen.go
@@ -131,6 +131,9 @@ func newImportSpec(name string) *ast.ImportSpec {
 }
 
 func gen(wantImps []string) {
+	if nodes == nil {
+		return
+	}
 	of, err := os.Create("pgen.go")
 	if err != nil {
 		log.Fatal("Cannot open pgen.go")