diff --git a/statemonitor.lua b/statemonitor.lua
index 5536924..757cb5a 100644
--- a/statemonitor.lua
+++ b/statemonitor.lua
@@ -22,6 +22,8 @@ end
 function statemonitor.register(name, check, callback, frequency, once)
 	frequency=frequency or 5
 	once=ternary(once ~= nil, once, true)
+	assert(type(callback)=="function")
+	assert(type(check)=="function")
 	callbacks[name] = {check=check, callback=callback, f=frequency,
 	deregister=once}
 end