diff --git a/packages/state-machine/src/__tests__/machine.test.ts b/packages/state-machine/src/__tests__/machine.test.ts
index ff624e3..0d59d12 100644
--- a/packages/state-machine/src/__tests__/machine.test.ts
+++ b/packages/state-machine/src/__tests__/machine.test.ts
@@ -15,6 +15,7 @@ describe('ATM State Machine', () => {
       dispensed: true,
     } satisfies DispenseCashResult),
     getExchangeRate: vi.fn().mockResolvedValue(2500), // 2500 sats per USD
+    getAvailableBalance: vi.fn().mockResolvedValue(1_000_000), // 1M sats available
     // noffer cash-out services (legacy)
     generateNoffer: vi.fn().mockResolvedValue('noffer1atmtest'),
     sendOfferResponse: vi.fn().mockResolvedValue(undefined),
diff --git a/packages/state-machine/src/machine.ts b/packages/state-machine/src/machine.ts
index 4d448b2..3f7e438 100644
--- a/packages/state-machine/src/machine.ts
+++ b/packages/state-machine/src/machine.ts
@@ -368,7 +368,9 @@ export function createATMMachine(
       hasOfferRequest: ({ context }) => context.pendingOfferRequest !== null,
       billWithinBalance: ({ context, event }) => {
         if (event.type !== 'BILL_INSERTED') return false
-        if (context.availableBalance <= 0 || context.exchangeRate === 0) return true
+        // Reject bills if balance or rate is unknown — don't risk accepting
+        // cash we can't cover with sats
+        if (context.availableBalance <= 0 || context.exchangeRate === 0) return false
         const newFiatCents = context.fiatCents + event.denomination * 100
         const grossSats = Math.floor((newFiatCents / 100) * context.exchangeRate)
         const fee = Math.floor(grossSats * context.feePercent)