Polishing

spring-web/src/main/java/org/springframework/http/HttpHeaders.java

@@ -400,7 +400,6 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	 * Private constructor that can create read-only {@code HttpHeader} instances.
 	 */
 	private HttpHeaders(Map<String, List<String>> headers, boolean readOnly) {
-		Assert.notNull(headers, "'headers' must not be null");
 		if (readOnly) {
 			Map<String, List<String>> map =
 					new LinkedCaseInsensitiveMap<List<String>>(headers.size(), Locale.ENGLISH);
@@ -1319,6 +1318,7 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	 * Return a {@code HttpHeaders} object that can only be read, not written to.
 	 */
 	public static HttpHeaders readOnlyHttpHeaders(HttpHeaders headers) {
+		Assert.notNull(headers, "HttpHeaders must not be null");
 		return new HttpHeaders(headers, true);
 	}