diff --git a/artiq/coredevice/core.py b/artiq/coredevice/core.py
index 47ae94803..8f9330287 100644
--- a/artiq/coredevice/core.py
+++ b/artiq/coredevice/core.py
@@ -38,7 +38,7 @@ class CompileError(Exception):
 
 
 @syscall
-def rtio_init():
+def rtio_init() -> TNone:
     raise NotImplementedError("syscall not simulated")
 
 @syscall(flags={"nounwind", "nowrite"})