diff --git a/imageproxy.go b/imageproxy.go
index ac6b357..a4ca19e 100644
--- a/imageproxy.go
+++ b/imageproxy.go
@@ -142,27 +142,32 @@ func (p *Proxy) serveImage(w http.ResponseWriter, r *http.Request) {
 	cached := resp.Header.Get(httpcache.XFromCache)
 	glog.Infof("request: %v (served from cache: %v)", *req, cached == "1")
 
-	copyHeader(w, resp, "Cache-Control")
-	copyHeader(w, resp, "Last-Modified")
-	copyHeader(w, resp, "Expires")
-	copyHeader(w, resp, "Etag")
-	copyHeader(w, resp, "Link")
+	copyHeader(w.Header(), resp.Header, "Cache-Control", "Last-Modified", "Expires", "Etag", "Link")
 
 	if should304(r, resp) {
 		w.WriteHeader(http.StatusNotModified)
 		return
 	}
 
-	copyHeader(w, resp, "Content-Length")
-	copyHeader(w, resp, "Content-Type")
+	copyHeader(w.Header(), resp.Header, "Content-Length", "Content-Type")
 	w.WriteHeader(resp.StatusCode)
 	io.Copy(w, resp.Body)
 }
 
-func copyHeader(w http.ResponseWriter, r *http.Response, header string) {
-	key := http.CanonicalHeaderKey(header)
-	if value, ok := r.Header[key]; ok {
-		w.Header()[key] = value
+// copyHeader copies header values from src to dst, adding to any existing
+// values with the same header name.  If keys is not empty, only those header
+// keys will be copied.
+func copyHeader(dst, src http.Header, keys ...string) {
+	if len(keys) == 0 {
+		for k, _ := range src {
+			keys = append(keys, k)
+		}
+	}
+	for _, key := range keys {
+		k := http.CanonicalHeaderKey(key)
+		for _, v := range src[k] {
+			dst.Add(k, v)
+		}
 	}
 }
 
diff --git a/imageproxy_test.go b/imageproxy_test.go
index 425059e..6a242c7 100644
--- a/imageproxy_test.go
+++ b/imageproxy_test.go
@@ -24,10 +24,78 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"net/url"
+	"reflect"
 	"strings"
 	"testing"
 )
 
+func TestCopyHeader(t *testing.T) {
+	tests := []struct {
+		dst, src http.Header
+		keys     []string
+		want     http.Header
+	}{
+		// empty
+		{http.Header{}, http.Header{}, nil, http.Header{}},
+		{http.Header{}, http.Header{}, []string{}, http.Header{}},
+		{http.Header{}, http.Header{}, []string{"A"}, http.Header{}},
+
+		// nothing to copy
+		{
+			dst:  http.Header{"A": []string{"a1"}},
+			src:  http.Header{},
+			keys: nil,
+			want: http.Header{"A": []string{"a1"}},
+		},
+		{
+			dst:  http.Header{},
+			src:  http.Header{"A": []string{"a"}},
+			keys: []string{"B"},
+			want: http.Header{},
+		},
+
+		// copy headers
+		{
+			dst:  http.Header{},
+			src:  http.Header{"A": []string{"a"}},
+			keys: nil,
+			want: http.Header{"A": []string{"a"}},
+		},
+		{
+			dst:  http.Header{"A": []string{"a"}},
+			src:  http.Header{"B": []string{"b"}},
+			keys: nil,
+			want: http.Header{"A": []string{"a"}, "B": []string{"b"}},
+		},
+		{
+			dst:  http.Header{"A": []string{"a"}},
+			src:  http.Header{"B": []string{"b"}, "C": []string{"c"}},
+			keys: []string{"B"},
+			want: http.Header{"A": []string{"a"}, "B": []string{"b"}},
+		},
+		{
+			dst:  http.Header{"A": []string{"a1"}},
+			src:  http.Header{"A": []string{"a2"}},
+			keys: nil,
+			want: http.Header{"A": []string{"a1", "a2"}},
+		},
+	}
+
+	for _, tt := range tests {
+		// copy dst map
+		got := make(http.Header)
+		for k, v := range tt.dst {
+			got[k] = v
+		}
+
+		copyHeader(got, tt.src, tt.keys...)
+		if !reflect.DeepEqual(got, tt.want) {
+			t.Errorf("copyHeader(%v, %v, %v) returned %v, want %v", tt.dst, tt.src, tt.keys, got, tt.want)
+		}
+
+	}
+}
+
 func TestAllowed(t *testing.T) {
 	whitelist := []string{"good"}
 	key := []byte("c0ffee")