diff --git a/imageproxy.go b/imageproxy.go
index 02c07e8..b304ea1 100644
--- a/imageproxy.go
+++ b/imageproxy.go
@@ -273,17 +273,11 @@ func peekContentType(p *bufio.Reader) string {
 	return http.DetectContentType(byt)
 }
 
-// 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)
+// copyHeader copies values for specified headers from src to dst, adding to
+// any existing values with the same header name.
+func copyHeader(dst, src http.Header, headerNames ...string) {
+	for _, name := range headerNames {
+		k := http.CanonicalHeaderKey(name)
 		for _, v := range src[k] {
 			dst.Add(k, v)
 		}
diff --git a/imageproxy_test.go b/imageproxy_test.go
index c2d4b82..87c2f5d 100644
--- a/imageproxy_test.go
+++ b/imageproxy_test.go
@@ -62,30 +62,12 @@ func TestCopyHeader(t *testing.T) {
 		},
 
 		// 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 {