diff --git a/mongoose.c b/mongoose.c
index df4a2959..d68a1508 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -692,7 +692,7 @@ void mg_http_printf_chunk(struct mg_connection *c, const char *fmt, ...) {
 }
 
 void mg_http_write_chunk(struct mg_connection *c, const char *buf, size_t len) {
-  mg_printf(c, "%X\r\n", len);
+  mg_printf(c, "%lX\r\n", (unsigned long) len);
   mg_send(c, buf, len);
   mg_send(c, "\r\n", 2);
 }
diff --git a/src/http.c b/src/http.c
index 0711fc8c..3e71d49d 100644
--- a/src/http.c
+++ b/src/http.c
@@ -270,7 +270,7 @@ void mg_http_printf_chunk(struct mg_connection *c, const char *fmt, ...) {
 }
 
 void mg_http_write_chunk(struct mg_connection *c, const char *buf, size_t len) {
-  mg_printf(c, "%X\r\n", len);
+  mg_printf(c, "%lX\r\n", (unsigned long) len);
   mg_send(c, buf, len);
   mg_send(c, "\r\n", 2);
 }