diff --git a/workspaces/arborist/lib/case-insensitive-map.js b/workspaces/arborist/lib/case-insensitive-map.js
index afc6afbe0f98a..2820db298ea7a 100644
--- a/workspaces/arborist/lib/case-insensitive-map.js
+++ b/workspaces/arborist/lib/case-insensitive-map.js
@@ -13,7 +13,8 @@ module.exports = class CIMap extends Map {
   }
 
   #normKey (key) {
-    if (typeof key !== 'string') {
+    // ASCII keys without uppercase letters are already normalized.
+    if (typeof key !== 'string' || !/[A-Z\u0080-\uffff]/.test(key)) {
       return key
     }
     return key.normalize('NFKD').toLowerCase()
diff --git a/workspaces/arborist/test/case-insensitive-map.js b/workspaces/arborist/test/case-insensitive-map.js
index 7c63bd67b37ea..04c9123f6cd1d 100644
--- a/workspaces/arborist/test/case-insensitive-map.js
+++ b/workspaces/arborist/test/case-insensitive-map.js
@@ -45,3 +45,39 @@ t.test('do not get confused with undefined or weird values', t => {
   ])
   t.end()
 })
+
+t.test('preserves case folding and Unicode normalization', t => {
+  for (const [key, equivalent] of [
+    ['@scope/package-name_1.0', '@SCOPE/PACKAGE-NAME_1.0'],
+    ['caf\u00e9', 'cafe\u0301'],
+    ['\u212a', 'k'],
+    ['\ufb03', 'ffi'],
+    ['\uff21', 'a'],
+    ['\ud835\udc00', 'a'],
+    ['\u0130', 'i\u0307'],
+  ]) {
+    const cmap = new CMap([[key, 1]])
+    t.equal(cmap.get(equivalent), 1)
+    t.equal(cmap.has(equivalent), true)
+    cmap.set(equivalent, 2)
+    t.equal(cmap.size, 1)
+    t.equal(cmap.get(key), 2)
+    t.strictSame([...cmap.keys()], [equivalent])
+    t.equal(cmap.delete(key), true)
+    t.equal(cmap.has(equivalent), false)
+  }
+  t.end()
+})
+
+t.test('preserves empty, undefined-valued, and non-string keys', t => {
+  const object = {}
+  const symbol = Symbol('key')
+  for (const key of ['', '\u0000', undefined, null, NaN, 0, object, symbol]) {
+    const cmap = new CMap([[key, undefined]])
+    t.equal(cmap.has(key), true)
+    t.equal(cmap.get(key), undefined)
+    t.equal(cmap.delete(key), true)
+    t.equal(cmap.has(key), false)
+  }
+  t.end()
+})