diff --git a/main/utils.c b/main/utils.c
index 3d3da881ca..d117174f64 100644
--- a/main/utils.c
+++ b/main/utils.c
@@ -1265,7 +1265,7 @@ void __ast_string_field_index_build_va(struct ast_string_field_mgr *mgr,
 	*/
 	if (fields[index][0] != '0') {
 		target = (char *) fields[index];
-		available = strlen(fields[index]);
+		available = strlen(fields[index]) + 1;
 	} else {
 		target = mgr->pool->base + mgr->used;
 		available = mgr->space;