diff --git a/src/runtime.h b/src/runtime.h
index ba579ea..b5c3608 100644
--- a/src/runtime.h
+++ b/src/runtime.h
@@ -142,11 +142,13 @@
 
   // Returns the "main" ThreadGroup, used when attaching user threads.
   jobject GetMainThreadGroup() const {
+    CHECK(main_thread_group_ != NULL || IsCompiler());
     return main_thread_group_;
   }
 
   // Returns the "system" ThreadGroup, used when attaching our internal threads.
   jobject GetSystemThreadGroup() const {
+    CHECK(system_thread_group_ != NULL || IsCompiler());
     return system_thread_group_;
   }