diff --git a/src/iris_diaphragm.py b/src/iris_diaphragm.py
index 6d5fd9099cc7030832ed608f450385047bb539fd..112078acc12f593cc2129de07843daa25d941c37 100755
--- a/src/iris_diaphragm.py
+++ b/src/iris_diaphragm.py
@@ -149,7 +149,7 @@ class IrisDiaphragm (PyTango.Device_4Impl):
         self.write_position(new_aperture)
 
     def is_Aperture_allowed(self, req_type):
-        return self.get_state() in (PyTango.DevState.ON,)
+        return req_type is PyTango.AttReqType.READ_REQ or self.get_state() in (PyTango.DevState.ON,)
 
     def read_StepSize(self, attr):
         # self.debug_stream("In read_StepSize()")