diff --git a/README.md b/README.md
index c357ae6..71f4f8e 100644
--- a/README.md
+++ b/README.md
@@ -123,6 +123,13 @@ Dates use strict `YYYY-MM-DD` calendar values, so impossible dates such as
 with code `INVALID_FIXTURE_SCHEMA` and a row plus field diagnostic; invalid
 amounts use `INVALID_FIXTURE_AMOUNT`.
 
+Collections may otherwise be empty, but scenarios that modify an existing
+entity have explicit minimums: `duplicate-invoice` requires at least one
+invoice and `vendor-bank-swap` requires at least one vendor. Missing scenario
+seed entities throw `LedgerpetError` with code
+`SCENARIO_FIXTURE_REQUIREMENT`; the other scenarios generate their own rows
+and do not require a non-empty collection.
+
 ## Development
 
 ```sh
diff --git a/src/scenarios.js b/src/scenarios.js
index 317156c..d283792 100644
--- a/src/scenarios.js
+++ b/src/scenarios.js
@@ -2,6 +2,11 @@ import { SCENARIOS } from "./constants.js";
 import { LedgerpetError } from "./errors.js";
 import { validateFixture } from "./fixtures.js";
 
+const SCENARIO_REQUIREMENTS = {
+  "duplicate-invoice": { collection: "invoices", entity: "invoice" },
+  "vendor-bank-swap": { collection: "vendors", entity: "vendor" }
+};
+
 export function listScenarios() {
   return [...SCENARIOS];
 }
@@ -11,6 +16,7 @@ export function generateScenario(fixture, scenario = "duplicate-invoice") {
     throw new LedgerpetError(`Unknown scenario '${scenario}'. Try one of: ${SCENARIOS.join(", ")}`, "UNKNOWN_SCENARIO");
   }
   validateFixture(fixture);
+  requireScenarioSeed(fixture, scenario);
   const clone = structuredClone(fixture);
   const findings = [];
   if (scenario === "duplicate-invoice") duplicateInvoice(clone, findings);
@@ -22,6 +28,16 @@ export function generateScenario(fixture, scenario = "duplicate-invoice") {
   return { fixture: clone, expectedFindings: findings };
 }
 
+function requireScenarioSeed(fixture, scenario) {
+  const requirement = SCENARIO_REQUIREMENTS[scenario];
+  if (requirement && fixture[requirement.collection].length === 0) {
+    throw new LedgerpetError(
+      `Scenario '${scenario}' requires at least one ${requirement.entity} in the fixture`,
+      "SCENARIO_FIXTURE_REQUIREMENT"
+    );
+  }
+}
+
 function duplicateInvoice(fixture, findings) {
   const invoice = fixture.invoices.find((row) => row.invoice_id === "INV-1003") ?? fixture.invoices[0];
   const duplicate = { ...invoice, invoice_id: `${invoice.invoice_id}-DUP`, due_date: shiftDate(invoice.due_date, 2), note: "Injected duplicate invoice" };
diff --git a/tests/scenarios.test.js b/tests/scenarios.test.js
index 2c5c003..2f22ce3 100644
--- a/tests/scenarios.test.js
+++ b/tests/scenarios.test.js
@@ -15,3 +15,25 @@ test("unknown scenario fails loudly", async () => {
   const base = await loadFixture("fixtures/sample");
   assert.throws(() => generateScenario(base, "nope"), /Unknown scenario/);
 });
+
+test("duplicate-invoice requires a seed invoice", async () => {
+  const base = await loadFixture("fixtures/sample");
+  base.invoices = [];
+  assert.throws(
+    () => generateScenario(base, "duplicate-invoice"),
+    (error) => error.name === "LedgerpetError" &&
+      error.code === "SCENARIO_FIXTURE_REQUIREMENT" &&
+      error.message === "Scenario 'duplicate-invoice' requires at least one invoice in the fixture"
+  );
+});
+
+test("vendor-bank-swap requires a seed vendor", async () => {
+  const base = await loadFixture("fixtures/sample");
+  base.vendors = [];
+  assert.throws(
+    () => generateScenario(base, "vendor-bank-swap"),
+    (error) => error.name === "LedgerpetError" &&
+      error.code === "SCENARIO_FIXTURE_REQUIREMENT" &&
+      error.message === "Scenario 'vendor-bank-swap' requires at least one vendor in the fixture"
+  );
+});