diff --git a/ext/descriptor/index.js b/ext/descriptor/index.js
index 6aafd2ab8..facef58ab 100644
--- a/ext/descriptor/index.js
+++ b/ext/descriptor/index.js
@@ -485,7 +485,7 @@ Field.prototype.toDescriptor = function toDescriptor(syntax) {
 
     // Handle part of oneof
     if (this.partOf)
-        if ((descriptor.oneofIndex = this.parent.oneofsArray.indexOf(this.partOf)) < 0)
+        if (this.parent.oneofsArray && (descriptor.oneofIndex = this.parent.oneofsArray.indexOf(this.partOf)) < 0)
             throw Error("missing oneof");
 
     if (this.options) {