diff --git a/clang/include/clang/Frontend/ASTUnit.h b/clang/include/clang/Frontend/ASTUnit.h
index 041eabb5b992..c22a18bd1c03 100644
--- a/clang/include/clang/Frontend/ASTUnit.h
+++ b/clang/include/clang/Frontend/ASTUnit.h
@@ -396,7 +396,9 @@ private:
   /// just about any usage.
   /// Becomes a noop in release mode; only useful for debug mode checking.
   class ConcurrencyState {
+#ifndef NDEBUG
     void *Mutex; // a llvm::sys::MutexImpl in debug;
+#endif
 
   public:
     ConcurrencyState();