diff --git a/panda/src/express/trueClock.cxx b/panda/src/express/trueClock.cxx
index 686bb59910..077af04c29 100644
--- a/panda/src/express/trueClock.cxx
+++ b/panda/src/express/trueClock.cxx
@@ -583,7 +583,7 @@ get_short_raw_time() {
 //  Description: 
 ////////////////////////////////////////////////////////////////////
 bool TrueClock::
-set_cpu_affinity(PN_int32 mask) {
+set_cpu_affinity(PN_int32 mask) const {
   return false;
 }