diff --git a/routstr/payment/price.py b/routstr/payment/price.py
index ae162398..e63d6754 100644
--- a/routstr/payment/price.py
+++ b/routstr/payment/price.py
@@ -12,6 +12,8 @@ logger = get_logger(__name__)
 BTC_USD_PRICE: float | None = None
 SATS_USD_PRICE: float | None = None
 
+SATS_PER_BTC = 100_000_000
+
 
 def _parse_quote(raw: object, exchange: str) -> float | None:
     """Coerce an exchange quote to a price, or ``None`` if it is not one.
@@ -20,10 +22,12 @@ def _parse_quote(raw: object, exchange: str) -> float | None:
     of what it collects: an unusable quote does not merely join the sample, it
     *wins* it, and the result is the rate every model and every request on the
     node is priced at. A quote is stricter than a billable rate — it must be
-    positive, since a BTC price of zero is a broken feed, not free money.
+    positive, and positive *after* the sats conversion the node prices in: a
+    subnormal quote survives every guard here and still underflows to a zero
+    sats price, which then divides by zero on every model's rate.
     """
     price = coerce_rate(raw)
-    if price is None or price <= 0:
+    if price is None or price <= 0 or price / SATS_PER_BTC <= 0:
         logger.warning(
             "Unusable price quote — ignoring this exchange",
             extra={"exchange": exchange, "quote": repr(raw)},
@@ -141,7 +145,7 @@ async def _update_prices() -> None:
         )
         return
     BTC_USD_PRICE = btc_price
-    SATS_USD_PRICE = btc_price / 100_000_000
+    SATS_USD_PRICE = btc_price / SATS_PER_BTC
 
 
 def btc_usd_price() -> float:
diff --git a/tests/unit/test_pricing_rate_validation.py b/tests/unit/test_pricing_rate_validation.py
index f0390c9b..895fa06e 100644
--- a/tests/unit/test_pricing_rate_validation.py
+++ b/tests/unit/test_pricing_rate_validation.py
@@ -315,9 +315,7 @@ async def test_boolean_exchange_quote_does_not_set_the_node_price(
 
     ``float(True)`` is ``1.0``, which is finite and positive, so a payload whose
     price field turned into a boolean passes every numeric guard — and then
-    *wins* the ``min()``, pricing the whole node at one dollar per bitcoin. The
-    node's other coercions all reject ``bool`` before the numeric check for this
-    reason; this one is the exception.
+    *wins* the ``min()``, pricing the whole node at one dollar per bitcoin.
     """
     from routstr.payment.price import btc_usd_price
 
@@ -328,6 +326,25 @@ async def test_boolean_exchange_quote_does_not_set_the_node_price(
     assert btc_usd_price() == pytest.approx(100000.0)
 
 
+@pytest.mark.asyncio
+async def test_an_underflowing_exchange_quote_does_not_set_the_node_price(
+    refresh_price_with: Any,
+) -> None:
+    """A quote too small to survive the sats conversion is not a price.
+
+    ``1e-320`` is positive, so it passes the guards and wins the ``min()``, but
+    the node prices in sats and ``1e-320 / 100_000_000`` underflows to ``0.0``
+    — a zero sats price divides by zero on every model's rate.
+    """
+    from routstr.payment.price import btc_usd_price
+
+    await refresh_price_with(
+        {"kraken": "1e-320", "coinbase": "100000.0", "binance": "100000.0"}
+    )
+
+    assert btc_usd_price() == pytest.approx(100000.0)
+
+
 @pytest.mark.asyncio
 async def test_an_unreadable_exchange_response_drops_only_that_quote(
     refresh_price_with: Any,