diff --git a/lib/contextify.js b/lib/contextify.js
index 49e404a..8171202 100644
--- a/lib/contextify.js
+++ b/lib/contextify.js
@@ -1076,6 +1076,8 @@ if (typeof OriginalCallSite === 'function') {
 	(function setupCallSite() {
 		for (let i=0; i<callSiteGetters.length; i++) {
 			const name = callSiteGetters[i].name;
+			const funcProp = Object.getOwnPropertyDescriptor(OriginalCallSite.prototype, name);
+			if (funcProp === undefined) continue;
 			const propertyName = callSiteGetters[i].propName;
 			const func = {func() {
 				return this[propertyName];
@@ -1083,7 +1085,6 @@ if (typeof OriginalCallSite === 'function') {
 			const nameProp = Object.getOwnPropertyDescriptor(func, 'name');
 			nameProp.value = name;
 			Object.defineProperty(func, 'name', nameProp);
-			const funcProp = Object.getOwnPropertyDescriptor(OriginalCallSite.prototype, name);
 			funcProp.value = func;
 			Object.defineProperty(CallSite.prototype, name, funcProp);
 		}