diff --git a/lib/util/cache.js b/lib/util/cache.js
index a8f112168e6..c88db6292b3 100644
--- a/lib/util/cache.js
+++ b/lib/util/cache.js
@@ -42,15 +42,57 @@ function normalizeHeaders (opts) {
     headers = {}
 
     if (hasSafeIterator(opts.headers)) {
-      for (const x of opts.headers) {
-        if (!Array.isArray(x)) {
-          throw new Error('opts.headers is not a valid header map')
+      if (Array.isArray(opts.headers)) {
+        // Array format: could be flat alternating [k, v, k, v, ...]
+        //  or array-of-pairs [[k, v], ...]
+        const first = opts.headers[0]
+        if (Array.isArray(first)) {
+          for (const x of opts.headers) {
+            if (!Array.isArray(x)) {
+              throw new Error('opts.headers is not a valid header map')
+            }
+            const [key, val] = x
+            if (typeof key !== 'string' || typeof val !== 'string') {
+              throw new Error('opts.headers is not a valid header map')
+            }
+            headers[key.toLowerCase()] = val
+          }
+        } else {
+          // Flat alternating array [k, v, k, v, ...]
+          const len = opts.headers.length
+          if (len % 2 !== 0) {
+            throw new Error('opts.headers is not a valid header map')
+          }
+          for (let i = 0; i < len; i += 2) {
+            const key = opts.headers[i]
+            const val = opts.headers[i + 1]
+            if (typeof key !== 'string' || (typeof val !== 'string' && !Array.isArray(val))) {
+              throw new Error('opts.headers is not a valid header map')
+            }
+            if (typeof val === 'string') {
+              headers[key.toLowerCase()] = val
+            } else {
+              const mapped = []
+              for (let j = 0; j < val.length; j++) {
+                const v = val[j]
+                mapped.push(typeof v === 'string' ? v : v.toString('latin1'))
+              }
+              headers[key.toLowerCase()] = mapped
+            }
+          }
         }
-        const [key, val] = x
-        if (typeof key !== 'string' || typeof val !== 'string') {
-          throw new Error('opts.headers is not a valid header map')
+      } else {
+        // Non-array iterable (e.g. Map) — use original iteration logic
+        for (const x of opts.headers) {
+          if (!Array.isArray(x)) {
+            throw new Error('opts.headers is not a valid header map')
+          }
+          const [key, val] = x
+          if (typeof key !== 'string' || typeof val !== 'string') {
+            throw new Error('opts.headers is not a valid header map')
+          }
+          headers[key.toLowerCase()] = val
         }
-        headers[key.toLowerCase()] = val
       }
     } else {
       for (const key of Object.keys(opts.headers)) {
diff --git a/test/cache-interceptor/cache-utils.js b/test/cache-interceptor/cache-utils.js
index 7b66e66d3aa..70f15958de4 100644
--- a/test/cache-interceptor/cache-utils.js
+++ b/test/cache-interceptor/cache-utils.js
@@ -42,3 +42,67 @@ test('normalizeHeaders handles headers from Map', (t) => {
 
   strictEqual(headers['x-test'], 'ok')
 })
+
+test('normalizeHeaders handles empty array', (t) => {
+  const { deepEqual } = tspl(t, { plan: 1 })
+
+  const headers = normalizeHeaders({ headers: [] })
+
+  deepEqual(headers, {})
+})
+
+test('normalizeHeaders handles flat alternating array (single header)', (t) => {
+  const { strictEqual } = tspl(t, { plan: 1 })
+
+  const headers = normalizeHeaders({
+    headers: ['host', 'localhost']
+  })
+
+  strictEqual(headers.host, 'localhost')
+})
+
+test('normalizeHeaders handles flat alternating array (multiple headers)', (t) => {
+  const { deepEqual } = tspl(t, { plan: 1 })
+
+  const headers = normalizeHeaders({
+    headers: ['host', 'localhost', 'content-type', 'application/json']
+  })
+
+  deepEqual(headers, { host: 'localhost', 'content-type': 'application/json' })
+})
+
+test('normalizeHeaders handles flat alternating array with array values', (t) => {
+  const { deepEqual } = tspl(t, { plan: 1 })
+
+  const headers = normalizeHeaders({
+    headers: ['accept', ['application/json', 'text/plain']]
+  })
+
+  deepEqual(headers, { accept: ['application/json', 'text/plain'] })
+})
+
+test('normalizeHeaders handles array-of-pairs (existing behavior)', (t) => {
+  const { strictEqual } = tspl(t, { plan: 1 })
+
+  const headers = normalizeHeaders({
+    headers: [['host', 'localhost']]
+  })
+
+  strictEqual(headers.host, 'localhost')
+})
+
+test('normalizeHeaders throws on odd-length flat array', (t) => {
+  const { throws } = require('node:assert')
+
+  throws(() => normalizeHeaders({ headers: ['host'] }), {
+    message: 'opts.headers is not a valid header map'
+  })
+})
+
+test('normalizeHeaders throws on non-string key in flat array', (t) => {
+  const { throws } = require('node:assert')
+
+  throws(() => normalizeHeaders({ headers: [42, 'value'] }), {
+    message: 'opts.headers is not a valid header map'
+  })
+})