diff --git a/lib/libwebsockets.h b/lib/libwebsockets.h
index 82479bf3..6e31aec9 100644
--- a/lib/libwebsockets.h
+++ b/lib/libwebsockets.h
@@ -4307,6 +4307,8 @@ LWS_VISIBLE LWS_EXTERN int LWS_WARN_UNUSED_RESULT
 lws_finalize_write_http_header(struct lws *wsi, unsigned char *start,
 			       unsigned char **p, unsigned char *end);
 
+#define LWS_ILLEGAL_HTTP_CONTENT_LEN ((lws_filepos_t)-1ll)
+
 /**
  * lws_add_http_common_headers() - Helper preparing common http headers
  *
@@ -4327,6 +4329,9 @@ lws_finalize_write_http_header(struct lws *wsi, unsigned char *start,
  * commonly needed.  If it doesn't fit your case, or you want to add additional
  * headers just call the public apis directly yourself for what you want.
  *
+ * You can miss out the content length header by providing the constant
+ * LWS_ILLEGAL_HTTP_CONTENT_LEN for the content_len.
+ *
  * It does not call lws_finalize_http_header(), to allow you to add further
  * headers after calling this.  You will need to call that yourself at the end.
  */
diff --git a/lib/roles/http/header.c b/lib/roles/http/header.c
index 1e17279b..48beb745 100644
--- a/lib/roles/http/header.c
+++ b/lib/roles/http/header.c
@@ -143,11 +143,14 @@ lws_add_http_common_headers(struct lws *wsi, unsigned int code,
 {
 	if (lws_add_http_header_status(wsi, code, p, end))
 		return 1;
+
 	if (lws_add_http_header_by_token(wsi, WSI_TOKEN_HTTP_CONTENT_TYPE,
 		    			(unsigned char *)content_type,
 		    			(int)strlen(content_type), p, end))
 		return 1;
-	if (lws_add_http_header_content_length(wsi, content_len, p, end))
+
+	if (content_len != LWS_ILLEGAL_HTTP_CONTENT_LEN &&
+	    lws_add_http_header_content_length(wsi, content_len, p, end))
 		return 1;
 
 	return 0;