Gracefully handle cases where no buy price was found

closes #4030

freqtrade/freqtradebot.py

@@ -616,6 +616,9 @@ class FreqtradeBot:
             # Calculate price
             buy_limit_requested = self.get_buy_rate(pair, True)
 
+        if not buy_limit_requested:
+            raise PricingError('Could not determine buy price.')
+
         min_stake_amount = self._get_min_pair_stake_amount(pair, buy_limit_requested)
         if min_stake_amount is not None and min_stake_amount > stake_amount:
             logger.warning(

tests/test_freqtradebot.py

@@ -1074,6 +1074,12 @@ def test_execute_buy(mocker, default_conf, fee, limit_buy_order, limit_buy_order
     mocker.patch('freqtrade.exchange.Exchange.buy', MagicMock(return_value=limit_buy_order))
     assert not freqtrade.execute_buy(pair, stake_amount)
 
+    # Fail to get price...
+    mocker.patch('freqtrade.freqtradebot.FreqtradeBot.get_buy_rate', MagicMock(return_value=0.0))
+
+    with pytest.raises(PricingError, match="Could not determine buy price."):
+        freqtrade.execute_buy(pair, stake_amount)
+
 
 def test_execute_buy_confirm_error(mocker, default_conf, fee, limit_buy_order) -> None:
     freqtrade = get_patched_freqtradebot(mocker, default_conf)