diff --git a/electrum/exchange_rate.py b/electrum/exchange_rate.py
index f1caabc5a..96403ca3f 100644
--- a/electrum/exchange_rate.py
+++ b/electrum/exchange_rate.py
@@ -46,7 +46,7 @@ class ExchangeBase(Logger):
 
     def __init__(self, on_quotes, on_history):
         Logger.__init__(self)
-        self._history = {}  # type: Dict[str, Dict[str, float]]
+        self._history = {}  # type: Dict[str, Dict[str, str | float]]
         self._quotes = {}  # type: Dict[str, Optional[Decimal]]
         self._quotes_timestamp = 0  # type: Union[int, float]
         self.on_quotes = on_quotes
@@ -101,7 +101,7 @@ class ExchangeBase(Logger):
     @staticmethod
     def _read_historical_rates_from_file(
         *, exchange_name: str, ccy: str, cache_dir: str,
-    ) -> Tuple[Optional[dict], Optional[float]]:
+    ) -> Tuple[Optional[Dict[str, str]], Optional[float]]:
         filename = os.path.join(cache_dir, f"{exchange_name}_{ccy}")
         if not os.path.exists(filename):
             return None, None
@@ -125,6 +125,7 @@ class ExchangeBase(Logger):
         )
         if not h:
             return None
+        assert timestamp is not None
         h['timestamp'] = timestamp
         self._history[ccy] = h
         self.on_history()
@@ -132,11 +133,17 @@ class ExchangeBase(Logger):
 
     @staticmethod
     def _write_historical_rates_to_file(
-        *, exchange_name: str, ccy: str, cache_dir: str, history: Dict[str, float],
+        *, exchange_name: str, ccy: str, cache_dir: str, history: Dict[str, str],
     ) -> None:
+        # sanity check types of history dict
+        assert 'timestamp' not in history
+        for key, rate in history.items():
+            assert isinstance(key, str), f"{exchange_name=}. {ccy=}. {key=!r}. {rate=!r}"
+            assert isinstance(rate, str), f"{exchange_name=}. {ccy=}. {key=!r}. {rate=!r}"
+        # write to file
         filename = os.path.join(cache_dir, f"{exchange_name}_{ccy}")
         with open(filename, 'w', encoding='utf-8') as f:
-            f.write(json.dumps(history))
+            f.write(json.dumps(history, sort_keys=True))
 
     @log_exceptions
     async def get_historical_rates_safe(self, ccy: str, cache_dir: str) -> None:
@@ -151,7 +158,7 @@ class ExchangeBase(Logger):
             self.logger.exception(f"failed fx history: {repr(e)}")
             return
         # cast rates to str
-        h_new = {date_str: str(rate) for (date_str, rate) in h_new.items()}
+        h_new = {date_str: str(rate) for (date_str, rate) in h_new.items()}  # type: Dict[str, str]
         # merge old history and new history. resolve duplicate dates using new data.
         h_old, _timestamp = self._read_historical_rates_from_file(
             exchange_name=self.name(), ccy=ccy, cache_dir=cache_dir,
@@ -162,7 +169,7 @@ class ExchangeBase(Logger):
         self._write_historical_rates_to_file(
             exchange_name=self.name(), ccy=ccy, cache_dir=cache_dir, history=h,
         )
-        h['timestamp'] = time.time()
+        h['timestamp'] = time.time()  # note: this is the only item in h that has a float value
         self._history[ccy] = h
         self.on_history()