diff --git a/fin_defs/__init__.py b/fin_defs/__init__.py
index a387a41..8321282 100644
--- a/fin_defs/__init__.py
+++ b/fin_defs/__init__.py
@@ -124,7 +124,8 @@ class Asset:
         if isinstance(self, Commodity):
             return f'commodity:{self.alpha_vantage_id}'
 
-        raise NotImplementedError('Unsupported asset type: ' + str(self))
+        msg = f'Unsupported asset type: {repr(self)}'
+        raise NotImplementedError(msg)
 
     @staticmethod
     def from_string_id(asset_id: str) -> 'Asset':
@@ -481,12 +482,15 @@ class AssetAmount:
         return self.amount < other.amount
 
 
+AssetPair = tuple[Asset, Asset]
+
+
 @dataclasses.dataclass(frozen=True, eq=True)
-class ExchangeRateSample(Asset):
+class ExchangeRateSample:
     """Single exchange rate sample with a timestamp and various stats."""
 
     timestamp: datetime.date | datetime.datetime
-    base_asset: Asset
+    asset_pair: AssetPair
     _average: Decimal | None = None
     last: Decimal | None = None
     open: Decimal | None = None
@@ -509,6 +513,7 @@ class ExchangeRateSample(Asset):
         one = Decimal('1')
         return dataclasses.replace(
             self,
+            asset_pair=(self.asset_pair[1], self.asset_pair[0]),
             _average=one / self._average if self._average else None,
             last=one / self.last if self.last else None,
             open=one / self.open if self.open else None,