diff --git a/lib/plugin.js b/lib/plugin.js
index e193d78..56a576c 100644
--- a/lib/plugin.js
+++ b/lib/plugin.js
@@ -2,6 +2,9 @@ import KY from 'ky-universal'
 
 class HTTP {
   constructor(defaults, ky = KY) {
+    const headers = defaults.headers
+    delete defaults.headers
+
     this._defaults = {
       hooks: {},
       headers: {},
@@ -9,9 +12,15 @@ class HTTP {
       ...defaults
     }
     this._ky = ky
+
+    for (const name in headers) {
+      this.setHeader(name, headers[name])
+    }
   }
 
   setHeader(name, value) {
+    name = name.toLowerCase()
+
     if (!value) {
       delete this._defaults.headers[name];
     } else {
diff --git a/test/fixture/pages/ssr.vue b/test/fixture/pages/ssr.vue
index 217ac36..d8771b1 100644
--- a/test/fixture/pages/ssr.vue
+++ b/test/fixture/pages/ssr.vue
@@ -12,7 +12,7 @@ let reqCtr = 1
 export default {
   computed: {
     httpSessionId() {
-      return this.$http._defaults.headers.sessionId
+      return this.$http._defaults.headers.sessionid
     },
 
     httpEncoding() {