diff --git a/fin_defs/__init__.py b/fin_defs/__init__.py
index 4b2a1c7..db93f0e 100644
--- a/fin_defs/__init__.py
+++ b/fin_defs/__init__.py
@@ -512,15 +512,26 @@ class AssetAmount:
     asset: Asset
     amount: Decimal
 
+    def __post_init__(self):
+        assert isinstance(self.asset, Asset), 'Incorrect value for self.asset'
+        assert isinstance(self.amount, Decimal), 'Incorrect value for self.amount'
+
     def __str__(self) -> str:
         return self.human_readable_str()
 
     def human_readable_str(self) -> str:
         abs_amount = abs(self.amount)
-        specificity = '2' if abs_amount >= THREE_DECIMALS_UNDER_THIS_AMOUNT else '3'
+
+        # Determine specificity
+        specificity = '3'
+        if abs_amount != abs_amount:
+            specificity = '0'
+        elif abs_amount >= THREE_DECIMALS_UNDER_THIS_AMOUNT:
+            specificity = '2'
+
         prefix = ASSET_PREFIX.get(self.asset, '')
         return ('{sign}{prefix}{amount:.' + specificity + 'f} {name}').format(
-            sign='-' if self.amount < 0 else '',
+            sign='-' if (self.amount == self.amount and self.amount < 0) else '',
             prefix=prefix,
             amount=abs_amount,
             name=self.asset.raw_short_name(),