Index: src/runtime/class.m
==================================================================
--- src/runtime/class.m
+++ src/runtime/class.m
@@ -928,10 +928,12 @@
 void
 objc_unregisterClass(Class class)
 {
 	static SEL unloadSel = NULL;
 
+	objc_globalMutex_lock();
+
 	if (unloadSel == NULL)
 		unloadSel = sel_registerName("unload");
 
 	while (class->subclassList != NULL && class->subclassList[0] != Nil)
 		objc_unregisterClass(class->subclassList[0]);
@@ -944,10 +946,12 @@
 	if (strcmp(class_getName(class), "Protocol") != 0)
 		classesCount--;
 
 	unregisterClass(class);
 	unregisterClass(class->isa);
+
+	objc_globalMutex_unlock();
 }
 
 void
 objc_unregisterAllClasses(void)
 {