diff --git a/.github/workflows/ci-build.yml b/.github/workflows/ci-build.yml
index 17321f5fd..93b2711e5 100644
--- a/.github/workflows/ci-build.yml
+++ b/.github/workflows/ci-build.yml
@@ -24,11 +24,12 @@ jobs:
     - uses: actions/checkout@v2
 
     - name: Install PHP
-      uses: shivammathur/setup-php@2.9.0
+      uses: shivammathur/setup-php@2.18.1
       with:
         php-version: ${{ matrix.php  }}
         coverage: none
         extensions: json, mbstring
+
     - name: Get Composer Cache Directory
       id: composer-cache
       run: echo "::set-output name=dir::$(composer config cache-files-dir)"
diff --git a/CHANGELOG.md b/CHANGELOG.md
index fe1fd7f5d..764e795ab 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,11 @@
 
 #### Unreleased
 
+#### 14.11.6
+
+Fixed:
+- Fix validation of modified sparse ASTs
+
 #### 14.11.5
 
 Fixed:
diff --git a/src/Validator/ValidationContext.php b/src/Validator/ValidationContext.php
index 549053eb5..0b25ff9a4 100644
--- a/src/Validator/ValidationContext.php
+++ b/src/Validator/ValidationContext.php
@@ -180,16 +180,15 @@ public function getFragmentSpreads(HasSelectionSet $node) : array
             while (count($setsToVisit) > 0) {
                 $set = array_pop($setsToVisit);
 
-                for ($i = 0, $selectionCount = count($set->selections); $i < $selectionCount; $i++) {
-                    $selection = $set->selections[$i];
+                foreach ($set->selections as $selection) {
                     if ($selection instanceof FragmentSpreadNode) {
                         $spreads[] = $selection;
-                    } elseif ($selection instanceof FieldNode || $selection instanceof InlineFragmentNode) {
-                        if ($selection->selectionSet) {
-                            $setsToVisit[] = $selection->selectionSet;
-                        }
                     } else {
-                        throw InvariantViolation::shouldNotHappen();
+                        assert($selection instanceof FieldNode || $selection instanceof InlineFragmentNode);
+                        $selectionSet = $selection->selectionSet;
+                        if ($selectionSet !== null) {
+                            $setsToVisit[] = $selectionSet;
+                        }
                     }
                 }
             }