diff --git a/fin_defs/__init__.py b/fin_defs/__init__.py
index cc4fbb6..97b1b3b 100644
--- a/fin_defs/__init__.py
+++ b/fin_defs/__init__.py
@@ -143,6 +143,8 @@ class Asset:
             return f'index:{self.ticker}'
         if isinstance(self, Commodity):
             return f'commodity:{self.alpha_vantage_id}'
+        if isinstance(self, UnknownAsset):
+            return 'unknown:XXX'
 
         msg = f'Unsupported asset type: {repr(self)}'
         raise ValueError(msg)
@@ -179,6 +181,8 @@ class Asset:
             return CryptoCurrency(ticker, **attrs)
         if prefix == 'commodity':
             return Commodity(ticker)
+        if prefix == 'unknown':
+            return UnknownAsset()
 
         msg = f'Unsupported asset format: {asset_id}'
         raise ValueError(msg)
@@ -192,6 +196,7 @@ class Asset:
 class UnknownAsset(Asset):
     pass
 
+
 @enforce_typing.enforce_types
 @dataclasses.dataclass(frozen=True)
 class Currency(Asset):
@@ -513,6 +518,10 @@ class AssetAmount:
         if not isinstance(other, AssetAmount):
             msg = f'other must be AssetAmount, but was {type(other)}'
             raise TypeError(msg)
+        if self.is_zero():
+            return other
+        if other.is_zero():
+            return self
         if self.asset != other.asset:
             msg = (
                 f'AssetAmount must have same asset, but: {self.asset} != {other.asset}'
@@ -525,6 +534,10 @@ class AssetAmount:
         if not isinstance(other, AssetAmount):
             msg = f'other must be AssetAmount, but was {type(other)}'
             raise TypeError(msg)
+        if self.is_zero():
+            return -other
+        if other.is_zero():
+            return self
         if self.asset != other.asset:
             msg = (
                 f'AssetAmount must have same asset, but: {self.asset} != {other.asset}'