diff --git a/net-dhcp/interface.py b/net-dhcp/interface.py
index f7af6fa..dec5c96 100644
--- a/net-dhcp/interface.py
+++ b/net-dhcp/interface.py
@@ -27,6 +27,8 @@ def GetInterfaces(library):
     elif library == 'IPR':
         for itf in iproute.get_links():
             interfaces.append(Interface(itf))
+    else:
+        raise NetDhcpError(f'No Interface Library selected')
     return interfaces
 
 def GetInterface(library, ifname):
@@ -34,6 +36,8 @@ def GetInterface(library, ifname):
         return Interface(ndb.interfaces[ifname])
     elif library == 'IPR':
         return Interface(iproute.get_links(ifname=ifname)[0])
+    else:
+        raise NetDhcpError(f'No Interface Library selected')
     return None
 
 class Interface:
@@ -52,6 +56,8 @@ class Interface:
             self.ipr = interface
         elif isinstance(interface, pyroute2.ndb.objects.interface.Interface):
             self.ndb = interface
+        else:
+            raise NetDhcpError(f'Unknown Interface type')
 
     def _initialize_attributes_(self):
         if self.ndb:
@@ -103,6 +109,8 @@ class Interface:
                     route['gateway'] = attrs['RTA_GATEWAY'] if 'RTA_GATEWAY' in attrs else None
                     route['dst_len'] = r['dst_len']
                     self.routes.append(route)
+        else:
+            raise NetDhcpError(f'No Interface Library selected')
 
     def SetAddress(self, address):
         self.address = address
@@ -110,18 +118,24 @@ class Interface:
              self.ndb.set('address', self.address).commit()
         elif self.ipr:
              iproute.link('set', index=self.index, address=self.address)
+        else:
+            raise NetDhcpError(f'No Interface Library selected')
 
     def Up(self):
         if self.ndb:
              self.ndb.set('state', 'up').commit()
         elif self.ipr:
              iproute.link("set", index=self.index, state="up")
+        else:
+            raise NetDhcpError(f'No Interface Library selected')
 
     def Down(self):
         if self.ndb:
              self.ndb.set('state', 'down').commit()
         elif self.ipr:
              iproute.link("set", index=self.index, state="down")
+        else:
+            raise NetDhcpError(f'No Interface Library selected')
 
 def CreateInterface(library, ifname, kind, peer):
     try:
@@ -131,6 +145,8 @@ def CreateInterface(library, ifname, kind, peer):
         elif library == 'IPR':
             iproute.link("add", ifname=ifname, kind=kind, peer=peer)
             return Interface(iproute.get_links(ifname=ifname)[0])
+        else:
+            raise NetDhcpError(f'No Interface Library selected')
         return None
     except Exception as e:
         return None
@@ -141,6 +157,8 @@ def RemoveInterface(library, ifname):
             ndb.interfaces[ifname].remove().commit()
         elif library == 'IPR':
             iproute.link("del", index=iproute.link_lookup(ifname=ifname)[0])
+        else:
+            raise NetDhcpError(f'No Interface Library selected')
     except Exception as e:
         return
 
@@ -150,6 +168,8 @@ def AddPort(library, bridge, ifname):
             ndb.interfaces[bridge].add_port(ifname).commit()
         elif library == 'IPR':
             iproute.link("set", index=iproute.link_lookup(ifname=ifname)[0], master=iproute.link_lookup(ifname=bridge)[0])
+        else:
+            raise NetDhcpError(f'No Interface Library selected')
     except Exception as e:
         return
 
@@ -159,6 +179,8 @@ def DelPort(library, bridge, ifname):
             ndb.interfaces[bridge].del_port(ifname).commit()
         elif library == 'IPR':
             iproute.link("set", index=iproute.link_lookup(ifname=ifname)[0], master=0)
+        else:
+            raise NetDhcpError(f'No Interface Library selected')
     except Exception as e:
         return