diff --git a/src/chrono.cpp b/src/chrono.cpp
index 73c83ee..1ce2e28 100644
--- a/src/chrono.cpp
+++ b/src/chrono.cpp
@@ -24,6 +24,8 @@
 
 // system_clock
 
+const bool system_clock::is_steady;
+
 system_clock::time_point
 system_clock::now() _NOEXCEPT
 {
@@ -46,6 +48,8 @@
 
 // steady_clock
 
+const bool steady_clock::is_steady;
+
 #if __APPLE__
 //   mach_absolute_time() * MachInfo.numer / MachInfo.denom is the number of
 //   nanoseconds since the computer booted up.  MachInfo.numer and MachInfo.denom