diff --git a/Source/ASTextNode.mm b/Source/ASTextNode.mm
index e98dfb261..5452f96fd 100644
--- a/Source/ASTextNode.mm
+++ b/Source/ASTextNode.mm
@@ -1324,8 +1324,9 @@ + (id)allocWithZone:(struct _NSZone *)zone
   // We are descended from ASTextNode. We need to change the superclass for the
   // ASTextNode subclass to ASTextNode2.
   // Walk up the class hierarchy until we find ASTextNode.
+  // Note: This may be called on multiple threads simultaneously.
   Class s;
-  for (Class c = self; c != [ASTextNode class]; c = s) {
+  for (Class c = self; c != Nil && c != [ASTextNode class]; c = s) {
     s = class_getSuperclass(c);
     if (s == [ASTextNode class]) {
 #pragma clang diagnostic push