diff --git a/packages/vite/src/node/__tests__/config.spec.ts b/packages/vite/src/node/__tests__/config.spec.ts
index 6d4102830badda..dd427c19433475 100644
--- a/packages/vite/src/node/__tests__/config.spec.ts
+++ b/packages/vite/src/node/__tests__/config.spec.ts
@@ -141,6 +141,22 @@ describe('mergeConfig', () => {
 
     expect(mergeConfig(baseConfig, newConfig)).toEqual(mergedConfig)
   })
+
+  test('merge array correctly', () => {
+    const baseConfig = {
+      foo: null
+    }
+
+    const newConfig = {
+      foo: ['bar']
+    }
+
+    const mergedConfig = {
+      foo: ['bar']
+    }
+
+    expect(mergeConfig(baseConfig, newConfig)).toEqual(mergedConfig)
+  })
 })
 
 describe('resolveConfig', () => {
diff --git a/packages/vite/src/node/config.ts b/packages/vite/src/node/config.ts
index 1313a16c01df9a..2b55f7355a6ca2 100644
--- a/packages/vite/src/node/config.ts
+++ b/packages/vite/src/node/config.ts
@@ -731,21 +731,24 @@ function mergeConfigRecursively(
 
     const existing = merged[key]
 
+    if (existing == null) {
+      merged[key] = value
+      continue
+    }
+
     // fields that require special handling
-    if (existing != null) {
-      if (key === 'alias' && (rootPath === 'resolve' || rootPath === '')) {
-        merged[key] = mergeAlias(existing, value)
-        continue
-      } else if (key === 'assetsInclude' && rootPath === '') {
-        merged[key] = [].concat(existing, value)
-        continue
-      } else if (key === 'noExternal' && existing === true) {
-        continue
-      }
+    if (key === 'alias' && (rootPath === 'resolve' || rootPath === '')) {
+      merged[key] = mergeAlias(existing, value)
+      continue
+    } else if (key === 'assetsInclude' && rootPath === '') {
+      merged[key] = [].concat(existing, value)
+      continue
+    } else if (key === 'noExternal' && existing === true) {
+      continue
     }
 
     if (Array.isArray(existing) || Array.isArray(value)) {
-      merged[key] = [...arraify(existing), ...arraify(value)]
+      merged[key] = [...arraify(existing ?? []), ...arraify(value ?? [])]
       continue
     }
     if (isObject(existing) && isObject(value)) {