diff --git a/ast.go b/ast.go
index 2899888..5394d80 100644
--- a/ast.go
+++ b/ast.go
@@ -83,8 +83,9 @@ func (self *ast) insert(tree, node *term) (root *term, err error) {
 		if tree.isComplete() {
 			var subRoot *term
 			last := tree.removeLastChild()
-			subRoot, err = self.insert(last, node)
-			subRoot.setParent(tree)
+			if subRoot, err = self.insert(last, node); err == nil {
+				subRoot.setParent(tree)
+			}
 		} else {
 			node.setParent(tree)
 		}