fix #3783

lib/exchange_rate.py

@@ -5,6 +5,7 @@ import sys
 from threading import Thread
 import time
 import csv
+import decimal
 from decimal import Decimal
 
 from .bitcoin import COIN
@@ -389,7 +390,11 @@ class FxThread(ThreadJob):
     def ccy_amount_str(self, amount, commas):
         prec = CCY_PRECISIONS.get(self.ccy, 2)
         fmt_str = "{:%s.%df}" % ("," if commas else "", max(0, prec))
-        return fmt_str.format(round(amount, prec))
+        try:
+            rounded_amount = round(amount, prec)
+        except decimal.InvalidOperation:
+            rounded_amount = amount
+        return fmt_str.format(rounded_amount)
 
     def run(self):
         # This runs from the plugins thread which catches exceptions