diff --git a/electrum/gui/kivy/main_window.py b/electrum/gui/kivy/main_window.py
index ba8e7da26..b5468bb7f 100644
--- a/electrum/gui/kivy/main_window.py
+++ b/electrum/gui/kivy/main_window.py
@@ -69,7 +69,8 @@ Label.register('Roboto',
 
 
 from electrum.util import (base_units, NoDynamicFeeEstimates, decimal_point_to_base_unit_name,
-                           base_unit_name_to_decimal_point, NotEnoughFunds)
+                           base_unit_name_to_decimal_point, NotEnoughFunds, UnknownBaseUnit,
+                           DECIMAL_POINT_DEFAULT)
 
 
 class ElectrumWindow(App):
@@ -163,8 +164,11 @@ class ElectrumWindow(App):
         self._trigger_update_history()
 
     def _get_bu(self):
-        decimal_point = self.electrum_config.get('decimal_point', 5)
-        return decimal_point_to_base_unit_name(decimal_point)
+        decimal_point = self.electrum_config.get('decimal_point', DECIMAL_POINT_DEFAULT)
+        try:
+            return decimal_point_to_base_unit_name(decimal_point)
+        except UnknownBaseUnit:
+            return decimal_point_to_base_unit_name(DECIMAL_POINT_DEFAULT)
 
     def _set_bu(self, value):
         assert value in base_units.keys()
diff --git a/electrum/gui/qt/main_window.py b/electrum/gui/qt/main_window.py
index 756676034..4d2eb241a 100644
--- a/electrum/gui/qt/main_window.py
+++ b/electrum/gui/qt/main_window.py
@@ -49,7 +49,8 @@ from electrum.util import (format_time, format_satoshis, format_fee_satoshis,
                            UserCancelled, NoDynamicFeeEstimates, profiler,
                            export_meta, import_meta, bh2u, bfh, InvalidPassword,
                            base_units, base_units_list, base_unit_name_to_decimal_point,
-                           decimal_point_to_base_unit_name, quantize_feerate)
+                           decimal_point_to_base_unit_name, quantize_feerate,
+                           UnknownBaseUnit, DECIMAL_POINT_DEFAULT)
 from electrum.transaction import Transaction, TxOutput
 from electrum.address_synchronizer import AddTransactionException
 from electrum.wallet import Multisig_Wallet, CannotBumpFee
@@ -126,8 +127,12 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
         self.create_status_bar()
         self.need_update = threading.Event()
 
-        self.decimal_point = config.get('decimal_point', 5)
-        self.num_zeros     = int(config.get('num_zeros',0))
+        self.decimal_point = config.get('decimal_point', DECIMAL_POINT_DEFAULT)
+        try:
+            decimal_point_to_base_unit_name(self.decimal_point)
+        except UnknownBaseUnit:
+            self.decimal_point = DECIMAL_POINT_DEFAULT
+        self.num_zeros = int(config.get('num_zeros', 0))
 
         self.completions = QStringListModel()
 
diff --git a/electrum/util.py b/electrum/util.py
index 123dc2123..de72cc80d 100644
--- a/electrum/util.py
+++ b/electrum/util.py
@@ -49,13 +49,18 @@ base_units = {'BTC':8, 'mBTC':5, 'bits':2, 'sat':0}
 base_units_inverse = inv_dict(base_units)
 base_units_list = ['BTC', 'mBTC', 'bits', 'sat']  # list(dict) does not guarantee order
 
+DECIMAL_POINT_DEFAULT = 5  # mBTC
+
+
+class UnknownBaseUnit(Exception): pass
+
 
 def decimal_point_to_base_unit_name(dp: int) -> str:
     # e.g. 8 -> "BTC"
     try:
         return base_units_inverse[dp]
     except KeyError:
-        raise Exception('Unknown base unit')
+        raise UnknownBaseUnit(dp) from None
 
 
 def base_unit_name_to_decimal_point(unit_name: str) -> int:
@@ -63,7 +68,7 @@ def base_unit_name_to_decimal_point(unit_name: str) -> int:
     try:
         return base_units[unit_name]
     except KeyError:
-        raise Exception('Unknown base unit')
+        raise UnknownBaseUnit(unit_name) from None
 
 
 def normalize_version(v):