diff --git a/packages/core/src/skeleton.js b/packages/core/src/skeleton.js
index a0f38f7..983bde3 100644
--- a/packages/core/src/skeleton.js
+++ b/packages/core/src/skeleton.js
@@ -69,7 +69,7 @@ function Handler(cachedCaptures) {
 function classOf(v) {
   if (v instanceof Record) {
     return v.getConstructorInfo();
-  } else if (v instanceof Immutable.List) {
+  } else if (Immutable.List.isList(v)) {
     return v.size;
   } else {
     return null;