diff --git a/src/options.spec.ts b/src/options.spec.ts
new file mode 100644
index 000000000..f79902508
--- /dev/null
+++ b/src/options.spec.ts
@@ -0,0 +1,18 @@
+import { describe, expect, it } from 'vitest'
+import { resolveOptions } from './options'
+import { mockWarn } from '../tests/vitest-mock-warn'
+
+describe('options', () => {
+  mockWarn()
+  it('ensure starting dots in extensions', () => {
+    expect(
+      resolveOptions({
+        extensions: ['vue', '.ts'],
+      })
+    ).toMatchObject({
+      extensions: ['.vue', '.ts'],
+    })
+
+    expect('Invalid extension "vue"').toHaveBeenWarned()
+  })
+})
diff --git a/src/options.ts b/src/options.ts
index 47ca9788b..8680936f8 100644
--- a/src/options.ts
+++ b/src/options.ts
@@ -132,9 +132,22 @@ export function resolveOptions(options: Options): ResolvedOptions {
   }))
 
   if (options.extensions) {
-    // sort extensions by length to ensure that the longest one is used first
-    // e.g. ['.vue', '.page.vue'] -> ['.page.vue', '.vue'] as both would match and order matters
-    options.extensions.sort((a, b) => b.length - a.length)
+    options.extensions = options.extensions
+      // ensure that extensions start with a dot or warn the user
+      // this is needed when filtering the files with the pattern .{vue,js,ts}
+      // in src/index.ts
+      .map((ext) => {
+        if (!ext.startsWith('.')) {
+          console.warn(
+            `[unplugin-vue-router]: Invalid extension "${ext}". Extensions must start with a dot.`
+          )
+          return '.' + ext
+        }
+        return ext
+      })
+      // sort extensions by length to ensure that the longest one is used first
+      // e.g. ['.vue', '.page.vue'] -> ['.page.vue', '.vue'] as both would match and order matters
+      .sort((a, b) => b.length - a.length)
   }
 
   return {
diff --git a/tests/vitest-mock-warn.ts b/tests/vitest-mock-warn.ts
new file mode 100644
index 000000000..f75e6496f
--- /dev/null
+++ b/tests/vitest-mock-warn.ts
@@ -0,0 +1,118 @@
+// https://github.com/posva/jest-mock-warn/blob/master/src/index.js
+
+import { afterEach, beforeEach, expect, SpyInstance, vi } from 'vitest'
+
+export function mockWarn() {
+  expect.extend({
+    toHaveBeenWarned(received: string | RegExp) {
+      asserted.set(received.toString(), received)
+      const passed = warn.mock.calls.some((args) =>
+        typeof received === 'string'
+          ? args[0].indexOf(received) > -1
+          : received.test(args[0])
+      )
+      if (passed) {
+        return {
+          pass: true,
+          message: () => `expected "${received}" not to have been warned.`,
+        }
+      } else {
+        const msgs = warn.mock.calls.map((args) => args[0]).join('\n - ')
+        return {
+          pass: false,
+          message: () =>
+            `expected "${received}" to have been warned.\n\nActual messages:\n\n - ${msgs}`,
+        }
+      }
+    },
+
+    toHaveBeenWarnedLast(received: string | RegExp) {
+      asserted.set(received.toString(), received)
+      const lastCall = warn.mock.calls[warn.mock.calls.length - 1][0]
+      const passed =
+        typeof received === 'string'
+          ? lastCall.indexOf(received) > -1
+          : received.test(lastCall)
+      if (passed) {
+        return {
+          pass: true,
+          message: () => `expected "${received}" not to have been warned last.`,
+        }
+      } else {
+        const msgs = warn.mock.calls.map((args) => args[0]).join('\n - ')
+        return {
+          pass: false,
+          message: () =>
+            `expected "${received}" to have been warned last.\n\nActual messages:\n\n - ${msgs}`,
+        }
+      }
+    },
+
+    toHaveBeenWarnedTimes(received: string | RegExp, n: number) {
+      asserted.set(received.toString(), received)
+      let found = 0
+      warn.mock.calls.forEach((args) => {
+        const isFound =
+          typeof received === 'string'
+            ? args[0].indexOf(received) > -1
+            : received.test(args[0])
+        if (isFound) {
+          found++
+        }
+      })
+
+      if (found === n) {
+        return {
+          pass: true,
+          message: () =>
+            `expected "${received}" to have been warned ${n} times.`,
+        }
+      } else {
+        return {
+          pass: false,
+          message: () =>
+            `expected "${received}" to have been warned ${n} times but got ${found}.`,
+        }
+      }
+    },
+  })
+
+  let warn: SpyInstance
+  const asserted = new Map<string, string | RegExp>()
+
+  beforeEach(() => {
+    asserted.clear()
+    warn = vi.spyOn(console, 'warn')
+    warn.mockImplementation(() => {})
+  })
+
+  afterEach(() => {
+    const assertedArray = Array.from(asserted)
+    const nonAssertedWarnings = warn.mock.calls
+      .map((args) => args[0])
+      .filter((received) => {
+        return !assertedArray.some(([key, assertedMsg]) => {
+          return typeof assertedMsg === 'string'
+            ? received.indexOf(assertedMsg) > -1
+            : assertedMsg.test(received)
+        })
+      })
+    warn.mockRestore()
+    if (nonAssertedWarnings.length) {
+      nonAssertedWarnings.forEach((warning) => {
+        console.warn(warning)
+      })
+      throw new Error(`test case threw unexpected warnings.`)
+    }
+  })
+}
+
+declare global {
+  namespace Vi {
+    interface JestAssertion<T = any> {
+      toHaveBeenWarned(): void
+      toHaveBeenWarnedLast(): void
+      toHaveBeenWarnedTimes(n: number): void
+    }
+  }
+}