diff --git a/src/fastboot-headers.js b/src/fastboot-headers.js
index 1fe2549..1e4aea0 100644
--- a/src/fastboot-headers.js
+++ b/src/fastboot-headers.js
@@ -5,7 +5,14 @@ function FastBootHeaders(headers) {
   this.headers = {};
 
   for (var header in headers) {
-    this.headers[header] = headers[header].split(', ');
+    let value = headers[header];
+
+    // convert to array if not already
+    if (typeof value === 'string') {
+      value = value.split(', ');
+    }
+
+    this.headers[header] = value;
   }
 }
 
diff --git a/test/fastboot-headers-test.js b/test/fastboot-headers-test.js
index 10eb435..1f43a4e 100644
--- a/test/fastboot-headers-test.js
+++ b/test/fastboot-headers-test.js
@@ -68,6 +68,15 @@ describe('FastBootHeaders', function() {
     expect(headers.has('host')).to.be.false;
   });
 
+  it('handles header values that are already an array', function() {
+    var headers = {
+      'x-test-header': ['value1', 'value2']
+    };
+    headers = new FastBootHeaders(headers);
+
+    expect(headers.getAll('x-test-header')).to.deep.equal(['value1', 'value2']);
+  });
+
   it('appends entries onto a header, regardless of casing', function() {
     var headers = new FastBootHeaders();
 
@@ -149,4 +158,3 @@ describe('FastBootHeaders', function() {
     expect(entriesIterator.next()).to.deep.equal({ value: undefined, done: true });
   });
 });
-