diff --git a/artiq/coredevice/core.py b/artiq/coredevice/core.py
index 536a10054..79ba4d969 100644
--- a/artiq/coredevice/core.py
+++ b/artiq/coredevice/core.py
@@ -124,4 +124,6 @@ class Core:
 
     @kernel
     def break_realtime(self):
-        at_mu(syscall("rtio_get_counter") + 125000)
+        min_now = syscall("rtio_get_counter") + 125000
+        if now_mu() < min_now:
+            at_mu(min_now)