diff --git a/src/applications/diffusion/herald/HeraldCommitAdapter.php b/src/applications/diffusion/herald/HeraldCommitAdapter.php
--- a/src/applications/diffusion/herald/HeraldCommitAdapter.php
+++ b/src/applications/diffusion/herald/HeraldCommitAdapter.php
@@ -267,6 +267,11 @@
 
     $raw = $diff_file->loadFileData();
 
+    // See T13667. This happens when a commit is empty and affects no files.
+    if (!strlen($raw)) {
+      return false;
+    }
+
     $parser = new ArcanistDiffParser();
     $changes = $parser->parseDiff($raw);
 
@@ -290,6 +295,10 @@
       }
     }
 
+    if ($this->commitDiff === false) {
+      return array();
+    }
+
     if ($this->commitDiff instanceof Exception) {
       $ex = $this->commitDiff;
       $ex_class = get_class($ex);
diff --git a/src/applications/diffusion/query/pathchange/DiffusionPathChangeQuery.php b/src/applications/diffusion/query/pathchange/DiffusionPathChangeQuery.php
--- a/src/applications/diffusion/query/pathchange/DiffusionPathChangeQuery.php
+++ b/src/applications/diffusion/query/pathchange/DiffusionPathChangeQuery.php
@@ -88,7 +88,12 @@
       $change->setFileType($raw_change['fileType']);
       $change->setCommitIdentifier($commit->getCommitIdentifier());
 
-      $change->setTargetPath(ltrim($raw_change['targetPathName'], '/'));
+      $target_path = $raw_change['targetPathName'];
+      if ($target_path !== null) {
+        $target_path = ltrim($target_path, '/');
+      }
+      $change->setTargetPath($target_path);
+
       $change->setTargetCommitIdentifier($raw_change['targetCommitIdentifier']);
 
       $id = $raw_change['pathID'];
diff --git a/src/applications/diffusion/request/DiffusionRequest.php b/src/applications/diffusion/request/DiffusionRequest.php
--- a/src/applications/diffusion/request/DiffusionRequest.php
+++ b/src/applications/diffusion/request/DiffusionRequest.php
@@ -207,7 +207,7 @@
    */
   private function initializeFromDictionary(array $data) {
     $blob = idx($data, 'blob');
-    if (strlen($blob)) {
+    if (phutil_nonempty_string($blob)) {
       $blob = self::parseRequestBlob($blob, $this->supportsBranches());
       $data = $blob + $data;
     }
diff --git a/src/applications/macro/query/PhabricatorMacroQuery.php b/src/applications/macro/query/PhabricatorMacroQuery.php
--- a/src/applications/macro/query/PhabricatorMacroQuery.php
+++ b/src/applications/macro/query/PhabricatorMacroQuery.php
@@ -128,7 +128,7 @@
         $this->authorPHIDs);
     }
 
-    if (strlen($this->nameLike)) {
+    if (($this->nameLike !== null) && strlen($this->nameLike)) {
       $where[] = qsprintf(
         $conn,
         'm.name LIKE %~',
@@ -142,7 +142,7 @@
         $this->names);
     }
 
-    if (strlen($this->namePrefix)) {
+    if (($this->namePrefix !== null) && strlen($this->namePrefix)) {
       $where[] = qsprintf(
         $conn,
         'm.name LIKE %>',
diff --git a/src/applications/repository/query/PhabricatorRepositoryQuery.php b/src/applications/repository/query/PhabricatorRepositoryQuery.php
--- a/src/applications/repository/query/PhabricatorRepositoryQuery.php
+++ b/src/applications/repository/query/PhabricatorRepositoryQuery.php
@@ -633,7 +633,7 @@
         $this->uuids);
     }
 
-    if (strlen($this->datasourceQuery)) {
+    if (phutil_nonempty_string($this->datasourceQuery)) {
       // This handles having "rP" match callsigns starting with "P...".
       $query = trim($this->datasourceQuery);
       if (preg_match('/^r/', $query)) {
diff --git a/src/applications/repository/storage/PhabricatorRepository.php b/src/applications/repository/storage/PhabricatorRepository.php
--- a/src/applications/repository/storage/PhabricatorRepository.php
+++ b/src/applications/repository/storage/PhabricatorRepository.php
@@ -574,7 +574,7 @@
 
   public function getURI() {
     $short_name = $this->getRepositorySlug();
-    if (strlen($short_name)) {
+    if (phutil_nonempty_string($short_name)) {
       return "/source/{$short_name}/";
     }
 
@@ -862,7 +862,7 @@
 
   public function getDefaultBranch() {
     $default = $this->getDetail('default-branch');
-    if (strlen($default)) {
+    if (phutil_nonempty_string($default)) {
       return $default;
     }
 
diff --git a/src/infrastructure/markup/PhabricatorMarkupEngine.php b/src/infrastructure/markup/PhabricatorMarkupEngine.php
--- a/src/infrastructure/markup/PhabricatorMarkupEngine.php
+++ b/src/infrastructure/markup/PhabricatorMarkupEngine.php
@@ -583,6 +583,14 @@
     $engine->setConfig('viewer', $viewer);
 
     foreach ($content_blocks as $content_block) {
+      if ($content_block === null) {
+        continue;
+      }
+
+      if (!strlen($content_block)) {
+        continue;
+      }
+
       $engine->markupText($content_block);
       $phids = $engine->getTextMetadata(
         PhabricatorMentionRemarkupRule::KEY_MENTIONED,
diff --git a/src/infrastructure/markup/render.php b/src/infrastructure/markup/render.php
--- a/src/infrastructure/markup/render.php
+++ b/src/infrastructure/markup/render.php
@@ -105,6 +105,10 @@
 }
 
 function phutil_escape_html($string) {
+  if ($string === null) {
+    return '';
+  }
+
   if ($string instanceof PhutilSafeHTML) {
     return $string;
   } else if ($string instanceof PhutilSafeHTMLProducerInterface) {