diff --git a/src/system/PeriodClock.hxx b/src/system/PeriodClock.hxx
index 1ba74ece2..2c535fee9 100644
--- a/src/system/PeriodClock.hxx
+++ b/src/system/PeriodClock.hxx
@@ -63,7 +63,7 @@ protected:
   }
 
 public:
-  bool IsDefined() const {
+  constexpr bool IsDefined() const {
     return last != 0;
   }