diff --git a/src/utils/__tests__/PhutilUtilsTestCase.php b/src/utils/__tests__/PhutilUtilsTestCase.php
--- a/src/utils/__tests__/PhutilUtilsTestCase.php
+++ b/src/utils/__tests__/PhutilUtilsTestCase.php
@@ -352,6 +352,9 @@
     $this->assertEqual(null, idx($array, 'null', false));
     $this->assertEqual(null, idx($array, 'missing'));
     $this->assertEqual(false, idx($array, 'missing', false));
+
+    $array = new CaseInsensitiveArray();
+    $this->assertEqual(null, idx($array, 'key'));
   }
 
   public function testSplitLines() {
diff --git a/src/utils/utils.php b/src/utils/utils.php
--- a/src/utils/utils.php
+++ b/src/utils/utils.php
@@ -33,7 +33,14 @@
  * @return  wild    If `$array[$key]` exists, that value is returned. If not,
  *                  $default is returned without raising a warning.
  */
-function idx(array $array, $key, $default = null) {
+function idx($array, $key, $default = null) {
+  if (!(is_array($array) || $array instanceof ArrayAccess)) {
+    throw new InvalidArgumentException(
+      sprintf(
+        'Argument passed to %s must be an array.',
+        __FUNCTION__));
+  }
+
   // isset() is a micro-optimization - it is fast but fails for null values.
   if (isset($array[$key])) {
     return $array[$key];