diff --git a/src/monitor_utils/database.py b/src/monitor_utils/database.py
index 5c84188..246ca34 100644
--- a/src/monitor_utils/database.py
+++ b/src/monitor_utils/database.py
@@ -7,6 +7,7 @@ alert references for new products and price changes.
 """
 
 import logging
+import math
 from dataclasses import asdict
 from datetime import datetime, timezone
 from functools import cache
@@ -318,10 +319,13 @@ def calculate_priority(document: dict) -> int:
 
     Always recalculated from current and original price so the priority
     reflects the prices at alert time rather than any stale stored
-    percentage. Missing prices, zero original prices, and price increases
-    resolve to priority 0. A product with no MSRP has no percentage to rank
-    on and takes NO_MSRP_PRIORITY instead, since a real 0 would leave it
-    permanently behind every other alert in the claim query's sort. A new
+    percentage. Missing prices, zero original prices, non-finite prices, a
+    discount too large to represent, and price increases resolve to priority
+    0, so one product's bad pricing can never raise out of the whole cycle (a
+    retailer field like "Infinity" survives float() and used to crash the
+    monitor with an OverflowError). A product with no MSRP has no percentage
+    to rank on and takes NO_MSRP_PRIORITY instead, since a real 0 would leave
+    it permanently behind every other alert in the claim query's sort. A new
     listing is floored at NEW_LISTING_PRIORITY for the same reason: it may
     carry any discount at all, and its percentage rank still wins when higher.
 
@@ -337,10 +341,17 @@ def calculate_priority(document: dict) -> int:
     if current_price is None or not original_price:
         return 0
 
+    if not (math.isfinite(current_price) and math.isfinite(original_price)):
+        return 0
+
     if not document.get("hasMsrp", True):
         return NO_MSRP_PRIORITY
 
-    percentage = int(((original_price - current_price) / original_price) * 100)
+    discount = ((original_price - current_price) / original_price) * 100
+    if not math.isfinite(discount):
+        return 0
+
+    percentage = int(discount)
     priority = min(max(percentage // 10, 0), 10)
     if document.get("newListing"):
         return max(priority, NEW_LISTING_PRIORITY)
diff --git a/tests/test_alert_priority.py b/tests/test_alert_priority.py
index 19cd01e..968411f 100644
--- a/tests/test_alert_priority.py
+++ b/tests/test_alert_priority.py
@@ -50,6 +50,23 @@ def test_broken_prices_still_outrank_nothing():
     assert calculate_priority(document(None, 100.0, hasMsrp=False)) == 0
 
 
+def test_non_finite_prices_resolve_to_zero_instead_of_raising():
+    """float("Infinity") from a retailer field once crashed the whole monitor."""
+    inf, nan = float("inf"), float("nan")
+    assert calculate_priority(document(50.0, inf)) == 0
+    assert calculate_priority(document(inf, 100.0)) == 0
+    assert calculate_priority(document(-inf, 100.0)) == 0
+    assert calculate_priority(document(nan, 100.0)) == 0
+    assert calculate_priority(document(50.0, nan)) == 0
+    assert calculate_priority(document(50.0, inf, newListing=True)) == 0
+
+
+def test_a_discount_too_large_to_represent_resolves_to_zero():
+    """Finite prices can still overflow the ratio (a subnormal original)."""
+    assert calculate_priority(document(-1e308, 5e-324)) == 0
+    assert calculate_priority(document(1e308, 5e-324)) == 0
+
+
 def test_new_listing_is_floored_not_buried():
     """A low-discount new listing sends regardless, so it must not sort last."""
     assert (