From 280ffd8c692cc24199b678f38fc796825d7df4a1 Mon Sep 17 00:00:00 2001
From: Gleb Mazovetskiy <glex.spb@gmail.com>
Date: Sun, 25 Nov 2018 11:57:50 +0000
Subject: [PATCH] Fix crash in `selector-append('.x~~', 'a')`

Fixes #2663
---
 src/fn_selectors.cpp | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/src/fn_selectors.cpp b/src/fn_selectors.cpp
index e2e22e9e5..31388612a 100644
--- a/src/fn_selectors.cpp
+++ b/src/fn_selectors.cpp
@@ -140,11 +140,13 @@ namespace Sass {
 
             // TODO: Add check for namespace stuff
 
-            // append any selectors in childSeq's head
-            parentSeqClone->mutable_last()->head()->concat(base->head());
-
-            // Set parentSeqClone new tail
-            parentSeqClone->mutable_last()->tail( base->tail() );
+            Complex_Selector_Ptr lastComponent = parentSeqClone->mutable_last();
+            if (lastComponent->head() == nullptr) {
+              std::string msg = "Parent \"" + parentSeqClone->to_string() + "\" is incompatible with \"" + base->to_string() + "\"";
+              error(msg, pstate, traces);
+            }
+            lastComponent->head()->concat(base->head());
+            lastComponent->tail(base->tail());
 
             newElements.push_back(parentSeqClone);
           }