diff --git a/src/infrastructure/markup/markuprule/PhutilRemarkupHexColorCodeRule.php b/src/infrastructure/markup/markuprule/PhutilRemarkupHexColorCodeRule.php
--- a/src/infrastructure/markup/markuprule/PhutilRemarkupHexColorCodeRule.php
+++ b/src/infrastructure/markup/markuprule/PhutilRemarkupHexColorCodeRule.php
@@ -9,10 +9,13 @@
 
   public function apply($text) {
     // Match {#FFFFFF}
-    return preg_replace_callback(
-      '@\B\{(#([0-9a-fA-F]{3}){1,2})\}@',
-      array($this, 'markupHexColorCodedText'),
-      $text);
+    if (is_string($text)) {
+      return preg_replace_callback(
+        '@\B\{(#([0-9a-fA-F]{3}){1,2})\}@',
+        array($this, 'markupHexColorCodedText'),
+        $text);
+    }
+    return $text;
   }
 
   protected function contrastingColor($color_code) {
diff --git a/src/infrastructure/markup/markuprule/PhutilRemarkupRule.php b/src/infrastructure/markup/markuprule/PhutilRemarkupRule.php
--- a/src/infrastructure/markup/markuprule/PhutilRemarkupRule.php
+++ b/src/infrastructure/markup/markuprule/PhutilRemarkupRule.php
@@ -18,6 +18,12 @@
     return 500.0;
   }
 
+  /**
+   * Check input whether to apply RemarkupRule. If true, apply formatting.
+   * @param  string|PhutilSafeHTML String to check and potentially format.
+   * @return string|PhutilSafeHTML Unchanged input if no match, or input after
+   * matching the formatting rule and applying the formatting.
+   */
   abstract public function apply($text);
 
   public function getPostprocessKey() {