diff --git a/pywrparser/types/network.py b/pywrparser/types/network.py
index f540d59..4eac525 100644
--- a/pywrparser/types/network.py
+++ b/pywrparser/types/network.py
@@ -268,7 +268,7 @@ def attach_reference_parameters(self):
                     attached_parameters.append(value)
 
         for attached_parameter in attached_parameters:
-            del(self.parameters[attached_parameter])
+            self.parameters.pop(attached_parameter, None)
 
     def attach_reference_recorders(self):
         """
diff --git a/pywrparser/types/parameter.py b/pywrparser/types/parameter.py
index 2fb1970..77e2b7b 100644
--- a/pywrparser/types/parameter.py
+++ b/pywrparser/types/parameter.py
@@ -39,14 +39,6 @@ def rule_aggregated_has_paramlist(self):
         assert "parameters" in self.data and isinstance(self.data["parameters"], list),\
             f"AggregatedParameter <{self.name}> has invalid parameters"
 
-    @match("constantparameter")
-    def rule_constant_has_value(self):
-        assert "value" in self.data, f"ConstantParameter <{self.name}> does not define 'value'"
-
-    @match("controlcurveparameter")
-    def rule_cc_has_controlcurve(self):
-        assert "control_curve" in self.data, f"ControlCurveParameter <{self.name}> does not define 'control_curve'"
-
     @match("controlcurveparameter")
     def rule_cc_has_storage(self):
         assert "storage_node" in self.data, f"ControlCurveParameter <{self.name}> does not define 'storage_node'"
diff --git a/pywrparser/types/timestepper.py b/pywrparser/types/timestepper.py
index 5d165db..2334eb0 100644
--- a/pywrparser/types/timestepper.py
+++ b/pywrparser/types/timestepper.py
@@ -5,12 +5,12 @@ class PywrTimestepper(PywrType):
     def __init__(self, data):
         self.data = data
 
-
     """ Validation rules """
-
     def rule_start_required(self):
         assert "start" in self.data, "Timestepper does not define 'start' key"
 
-
     def rule_end_required(self):
         assert "end" in self.data, "Timestepper does not define 'end' key"
+
+    def rule_start_before_end(self):
+        pass