diff --git a/utils/xgboost_omp.h b/utils/xgboost_omp.h
index 3a8062b7e..ea1e7173c 100644
--- a/utils/xgboost_omp.h
+++ b/utils/xgboost_omp.h
@@ -10,7 +10,7 @@
 #if defined(_OPENMP)
 #include <omp.h>
 #else
-//#warning "OpenMP is not available, compile to single thread code"
+#warning "OpenMP is not available, compile to single thread code"
 inline int omp_get_thread_num() { return 0; }
 inline int omp_get_num_threads() { return 1; }
 inline void omp_set_num_threads(int nthread) {}