diff --git a/main/manager.c b/main/manager.c
index a02d06106d..1983ddeb20 100644
--- a/main/manager.c
+++ b/main/manager.c
@@ -4878,9 +4878,12 @@ static int generic_http_callback(struct ast_tcptls_session_instance *ser,
 
 	if (s.f != NULL) {	/* have temporary output */
 		char *buf;
-		size_t l = ftell(s.f);
+		size_t l;
 
-		if (l) {
+		/* Ensure buffer is NULL-terminated */
+		fprintf(s.f, "%c", 0);
+
+		if ((l = ftell(s.f))) {
 			if (MAP_FAILED == (buf = mmap(NULL, l, PROT_READ | PROT_WRITE, MAP_PRIVATE, s.fd, 0))) {
 				ast_log(LOG_WARNING, "mmap failed.  Manager output was not processed\n");
 			} else {