diff --git a/src/util/mpn.h b/src/util/mpn.h
index 495ae135c..f8cec0301 100644
--- a/src/util/mpn.h
+++ b/src/util/mpn.h
@@ -27,7 +27,9 @@ Revision History:
 typedef unsigned int mpn_digit;
 
 class mpn_manager {
+#ifndef _NO_OMP
     omp_nest_lock_t m_lock;
+#endif
 #define MPN_BEGIN_CRITICAL() omp_set_nest_lock(&m_lock);
 #define MPN_END_CRITICAL() omp_unset_nest_lock(&m_lock);