diff --git a/qcodes/instrument_drivers/rohde_schwarz/ZNB.py b/qcodes/instrument_drivers/rohde_schwarz/ZNB.py
index 19ff4497340..04cc552fc84 100644
--- a/qcodes/instrument_drivers/rohde_schwarz/ZNB.py
+++ b/qcodes/instrument_drivers/rohde_schwarz/ZNB.py
@@ -144,13 +144,12 @@ def __init__(self, parent: 'ZNB', name: str, channel: int, vna_parameter: str=No
         if full_modelname is not None:
             model = full_modelname.split('-')[0]
         else:
-            model = None
-        if model == 'ZNB4':
-            self._min_source_power = -80
-        elif model == 'ZNB8':
-            self._min_source_power = -80
-        elif model == 'ZNB20':
-            self._min_source_power = -60
+           raise RuntimeError("Could not determine ZNB model")
+        mSourcePower = {'ZNB4':-80, 'ZNB8':-80, 'ZNB20':-60}
+        if model not in mSourcePower.keys():
+            raise RuntimeError("Unsupported ZNB model: {}".format(model))
+        self._min_source_power: float
+        self._min_source_power = mSourcePower[model]
 
         self.add_parameter(name='vna_parameter',
                            label='VNA parameter',
@@ -425,17 +424,13 @@ def __init__(self, name: str, address: str, init_s_params: bool = True,
         else:
             raise RuntimeError("Could not determine ZNB model")
         # format seems to be ZNB8-4Port
-        if model == 'ZNB4':
-            self._max_freq = 4.5e9
-            self._min_freq = 9e3
-        elif model == 'ZNB8':
-            self._max_freq = 8.5e9
-            self._min_freq = 9e3
-        elif model == 'ZNB20':
-            self._max_freq = 20e9
-            self._min_freq = 100e3
-        else:
+        mFrequency = {'ZNB4':(9e3, 4.5e9), 'ZNB8':(9e3, 8.5e9), 'ZNB20':(100e3, 20e9)}
+        if model not in mFrequency.keys():
             raise RuntimeError("Unsupported ZNB model {}".format(model))
+        self._min_freq: float
+        self._max_freq: float
+        self._min_freq, self._max_freq = mFrequency[model]
+
         self.add_parameter(name='num_ports',
                            get_cmd='INST:PORT:COUN?',
                            get_parser=int)