Index: rvm/src/org/jikesrvm/scheduler/RVMThread.java
===================================================================
--- rvm/src/org/jikesrvm/scheduler/RVMThread.java	(revision 15406)
+++ rvm/src/org/jikesrvm/scheduler/RVMThread.java	(working copy)
@@ -1514,6 +1514,10 @@
   @NoInline
   @Unpreemptible("May block if the thread was asked to do so, but otherwise does no actions that would cause blocking")
   private void checkBlockNoSaveContext() {
+    assertUnacceptableStates(NEW, TERMINATED);
+    if (VM.VerifyAssertions) VM._assert(!isAboutToTerminate);
+    if (VM.VerifyAssertions) VM._assert(!isBlocking);
+
     if (traceBlock)
       VM.sysWriteln("Thread #", threadSlot, " in checkBlockNoSaveContext");
     // NB: anything this method calls CANNOT change the contextRegisters