diff --git a/jetty-http/src/main/java/org/eclipse/jetty/http/HttpFields.java b/jetty-http/src/main/java/org/eclipse/jetty/http/HttpFields.java
index 26d20e83ec2b..dbd1eecaa76f 100644
--- a/jetty-http/src/main/java/org/eclipse/jetty/http/HttpFields.java
+++ b/jetty-http/src/main/java/org/eclipse/jetty/http/HttpFields.java
@@ -642,7 +642,9 @@ public Mutable add(HttpFields fields)
             if (fields.size() == 0)
                 return this;
 
-            if (_size + fields.size() >= _fields.length)
+            if (_fields == null)
+                _fields = new HttpField[fields.size() + 4];
+            else if (_size + fields.size() >= _fields.length)
                 _fields = Arrays.copyOf(_fields, _size + fields.size() + 4);
 
             if (fields instanceof Immutable)
diff --git a/jetty-http/src/test/java/org/eclipse/jetty/http/HttpFieldsTest.java b/jetty-http/src/test/java/org/eclipse/jetty/http/HttpFieldsTest.java
index f31548e461c4..be2e871d40b1 100644
--- a/jetty-http/src/test/java/org/eclipse/jetty/http/HttpFieldsTest.java
+++ b/jetty-http/src/test/java/org/eclipse/jetty/http/HttpFieldsTest.java
@@ -747,6 +747,9 @@ public void testAddHttpFields()
     {
         HttpFields.Mutable fields = HttpFields.build();
         fields.add("One", "1");
+
+        fields = new HttpFields.Mutable(fields);
+
         fields.add(HttpFields.build().add("two", "2").add("three", "3"));
         fields.add(HttpFields.build().add("four", "4").add("five", "5").asImmutable());