diff --git a/lib/trading-logic.ts b/lib/trading-logic.ts
index 7a301c3..24a0534 100644
--- a/lib/trading-logic.ts
+++ b/lib/trading-logic.ts
@@ -1431,7 +1431,9 @@ export function sizeNotional(o: {
     for (let pass = 0; pass < 4; pass++) {
         const { a, b } = distances(n);
         const nTime = Math.sqrt((a * b) / (sigma * sigma * hold));
-        const nCost = fh > 0 ? Math.sqrt((fh * a * b) / (sigma * sigma * (2 * c + s))) : 0;
+        // No per-trade cost (a zero-commission firm on a zero spread): no
+        // floor — there is nothing for a bigger position to save.
+        const nCost = fh > 0 && 2 * c + s > 0 ? Math.sqrt((fh * a * b) / (sigma * sigma * (2 * c + s))) : 0;
         n = Math.max(nTime, nCost);
     }
     if (!Number.isFinite(n) || n <= 0) return null;