diff --git a/pjsip/src/pjsip/sip_multipart.c b/pjsip/src/pjsip/sip_multipart.c
index 7a91063..ef967ce 100644
--- a/pjsip/src/pjsip/sip_multipart.c
+++ b/pjsip/src/pjsip/sip_multipart.c
@@ -226,11 +226,11 @@
     if (ctype && ctype->type.slen) {
 	pjsip_media_type_cp(pool, &body->content_type, ctype);
     } else {
-	const pj_str_t STR_MULTIPART = {"multipart", 9};
-	const pj_str_t STR_MIXED = { "mixed", 5 };
+	pj_str_t STR_MULTIPART = {"multipart", 9};
+	pj_str_t STR_MIXED = { "mixed", 5 };
 
-	body->content_type.type = STR_MULTIPART;
-	body->content_type.subtype = STR_MIXED;
+        pjsip_media_type_init(&body->content_type,
+                              &STR_MULTIPART, &STR_MIXED);
     }
 
     /* multipart data */