diff --git a/src/field.js b/src/field.js
index 05d89cae8..20c1cd27d 100644
--- a/src/field.js
+++ b/src/field.js
@@ -78,13 +78,13 @@ function Field(name, id, type, rule, extend, options, comment) {
     if (extend !== undefined && !util.isString(extend))
         throw TypeError("extend must be a string");
 
+    if (rule === "proto3_optional") {
+        rule = "optional";
+    }
     /**
      * Field rule, if any.
      * @type {string|undefined}
      */
-    if (rule === "proto3_optional") {
-        rule = "optional";
-    }
     this.rule = rule && rule !== "optional" ? rule : undefined; // toJSON
 
     /**