diff --git a/core/qubes.py b/core/qubes.py
index 898b3ba2..375d666d 100755
--- a/core/qubes.py
+++ b/core/qubes.py
@@ -354,8 +354,11 @@ class QubesVmCollection(dict):
                               netvm = self.get_default_fw_netvm())
 
     def set_default_template(self, vm):
-        assert vm.is_template(), "VM {0} is not a TemplateVM!".format(vm.name)
-        self.default_template_qid = vm.qid
+        if vm is None:
+            self.default_template_qid = None
+        else:
+            assert vm.is_template(), "VM {0} is not a TemplateVM!".format(vm.name)
+            self.default_template_qid = vm.qid
 
     def get_default_template(self):
         if self.default_template_qid is None:
@@ -364,8 +367,11 @@ class QubesVmCollection(dict):
             return self[self.default_template_qid]
 
     def set_default_netvm(self, vm):
-        assert vm.is_netvm(), "VM {0} does not provide network!".format(vm.name)
-        self.default_netvm_qid = vm.qid
+        if vm is None:
+            self.default_netvm_qid = None
+        else:
+            assert vm.is_netvm(), "VM {0} does not provide network!".format(vm.name)
+            self.default_netvm_qid = vm.qid
 
     def get_default_netvm(self):
         if self.default_netvm_qid is None:
@@ -383,8 +389,11 @@ class QubesVmCollection(dict):
         return self.default_kernel
 
     def set_default_fw_netvm(self, vm):
-        assert vm.is_netvm(), "VM {0} does not provide network!".format(vm.name)
-        self.default_fw_netvm_qid = vm.qid
+        if vm is None:
+            self.default_fw_netvm_qid = None
+        else:
+            assert vm.is_netvm(), "VM {0} does not provide network!".format(vm.name)
+            self.default_fw_netvm_qid = vm.qid
 
     def get_default_fw_netvm(self):
         if self.default_fw_netvm_qid is None:
@@ -393,7 +402,10 @@ class QubesVmCollection(dict):
             return self[self.default_fw_netvm_qid]
 
     def set_updatevm_vm(self, vm):
-        self.updatevm_qid = vm.qid
+        if vm is None:
+            self.updatevm_qid = None
+        else:
+            self.updatevm_qid = vm.qid
 
     def get_updatevm_vm(self):
         if self.updatevm_qid is None:
@@ -402,7 +414,10 @@ class QubesVmCollection(dict):
             return self[self.updatevm_qid]
 
     def set_clockvm_vm(self, vm):
-        self.clockvm_qid = vm.qid
+        if vm is None:
+            self.clockvm_qid = None
+        else:
+            self.clockvm_qid = vm.qid
 
     def get_clockvm_vm(self):
         if self.clockvm_qid is None:
diff --git a/qvm-tools/qubes-prefs b/qvm-tools/qubes-prefs
index a8bb3b82..2c57562a 100755
--- a/qvm-tools/qubes-prefs
+++ b/qvm-tools/qubes-prefs
@@ -39,15 +39,22 @@ def handle_vm(vms, label, new_value = None):
     assert label in functions.keys()
 
     if new_value:
-        vm = vms.get_vm_by_name (new_value)
-        if vm is None:
-            print >> sys.stderr, "A VM with the name '{0}' does not exist in the system.".format(new_value)
-            exit(1)
-        try:
-            vms.__getattribute__(functions[label][1])(vm)
-        except Exception as e:
-            print >> sys.stderr, "ERROR: {0}".format(str(e))
-            exit(1)
+        if new_value == "none":
+            try:
+                vms.__getattribute__(functions[label][1])(None)
+            except Exception as e:
+                print >> sys.stderr, "ERROR: {0}".format(str(e))
+                exit(1)
+        else:
+            vm = vms.get_vm_by_name (new_value)
+            if vm is None:
+                print >> sys.stderr, "A VM with the name '{0}' does not exist in the system.".format(new_value)
+                exit(1)
+            try:
+                vms.__getattribute__(functions[label][1])(vm)
+            except Exception as e:
+                print >> sys.stderr, "ERROR: {0}".format(str(e))
+                exit(1)
     else:
         vm = vms.__getattribute__(functions[label][0])()
         if vm is not None: