diff --git a/search/Power.cc b/search/Power.cc
index 50174366..fa4ceb3a 100644
--- a/search/Power.cc
+++ b/search/Power.cc
@@ -63,6 +63,7 @@
 namespace sta {
 
 using std::abs;
+using std::isnormal;
 
 static bool
 isPositiveUnate(const LibertyCell *cell,