diff --git a/src/index.ts b/src/index.ts
index 87e4fea..33c2253 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -131,7 +131,7 @@ function externals(options: ExternalsOptions = {}): Plugin {
     const config: Config = Object.assign(Object.create(null), defaults, options)
     let include: RegExp[],
         exclude: RegExp[]
-    const isIncluded = (id: string) => include.length  === 0 || include.some(rx => rx.test(id))
+    const isIncluded = (id: string) => include.some(rx => rx.test(id))
     const isExcluded = (id: string) => exclude.some(rx => rx.test(id))
 
     return {
@@ -245,7 +245,7 @@ function externals(options: ExternalsOptions = {}): Plugin {
                 }
             }
 
-            // Handle npm dependencies.
+            // Handle other imports.
             return isIncluded(id) && !isExcluded(id)
                 ? false     // external
                 : null      // normal handling
diff --git a/test/relative.test.ts b/test/relative.test.ts
index 475cc71..5c1a7e9 100644
--- a/test/relative.test.ts
+++ b/test/relative.test.ts
@@ -1,26 +1,57 @@
 import test from 'ava'
 import { initPlugin, callHook } from './_common'
+import type { ExternalsOptions } from '../src/index'
 
-test("Filters out relative specifiers", async t => {
+const noDeps: ExternalsOptions = {
+    builtins: false,
+    deps: false,
+    devDeps: false,
+    optDeps: false,
+    peerDeps: false
+}
+
+test.only("Does NOT filter out relative specifiers by default", async t => {
+    const relativeSpecifiers = [ './sibling.js', '../parent.js' ]
+
+    const { plugin } = await initPlugin(noDeps)
+    for (const identifier of relativeSpecifiers) {
+        t.is(await callHook(plugin, 'resolveId', identifier), null)
+    }
+})
+
+test("Filters out relative specifiers when asked to", async t => {
     const relativeSpecifiers = [ './sibling.js', '../parent.js' ]
 
     const { plugin } = await initPlugin({
+        ...noDeps,
         include: relativeSpecifiers
     })
-    for (const builtin of relativeSpecifiers) {
-        t.false(await callHook(plugin, 'resolveId', builtin))
+    for (const identifier of relativeSpecifiers) {
+        t.false(await callHook(plugin, 'resolveId', identifier))
+    }
+})
+
+test("Does NOT filter out absolute specifiers by default", async t => {
+    const absoluteSpecifiers = [ '/root.js' ]
+    if (process.platform === 'win32')
+        absoluteSpecifiers.push('C:\\root.js', '\\root.js')
+
+    const { plugin } = await initPlugin(noDeps)
+    for (const identifier of absoluteSpecifiers) {
+        t.is(await callHook(plugin, 'resolveId', identifier), null, `Failed id: ${identifier}`)
     }
 })
 
-test("Does NOT filter out absolute specifiers", async t => {
+test("Filters out absolute specifiers when asked to", async t => {
     const absoluteSpecifiers = [ '/root.js' ]
     if (process.platform === 'win32')
         absoluteSpecifiers.push('C:\\root.js', '\\root.js')
 
     const { plugin } = await initPlugin({
+        ...noDeps,
         include: absoluteSpecifiers
     })
-    for (const builtin of absoluteSpecifiers) {
-        t.is(await callHook(plugin, 'resolveId', builtin), null)
+    for (const identifier of absoluteSpecifiers) {
+        t.is(await callHook(plugin, 'resolveId', identifier), null, `Failed id: ${identifier}`)
     }
 })