diff --git a/src/or/circuituse.c b/src/or/circuituse.c
index faad49c4e5..5488eb7bed 100644
--- a/src/or/circuituse.c
+++ b/src/or/circuituse.c
@@ -292,7 +292,7 @@ circuit_expire_building(void)
     long ms = tor_lround(msec);                             \
     struct timeval diff;                                    \
     diff.tv_sec = ms / 1000;                                \
-    diff.tv_usec = (ms % 1000) * 1000;                      \
+    diff.tv_usec = (int)((ms % 1000) * 1000);               \
     timersub(&now, &diff, &target);                         \
   } while (0)