diff --git a/src/__phutil_library_init__.php b/src/__phutil_library_init__.php
--- a/src/__phutil_library_init__.php
+++ b/src/__phutil_library_init__.php
@@ -26,10 +26,9 @@
         $class_name,
         pht('class or interface'),
         pht(
-          "the class or interface '%s' is not defined in the library ".
-          "map for any loaded %s library.",
-          $class_name,
-          'phutil'));
+          'The class or interface "%s" is not defined in the library '.
+          'map of any loaded library.',
+          $class_name));
     }
   } catch (PhutilMissingSymbolException $ex) {
     $should_throw = true;
diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -542,6 +542,7 @@
     'phutil_date_format' => 'utils/viewutils.php',
     'phutil_decode_mime_header' => 'utils/utils.php',
     'phutil_deprecated' => 'moduleutils/moduleutils.php',
+    'phutil_describe_type' => 'utils/utils.php',
     'phutil_error_listener_example' => 'error/phlog.php',
     'phutil_escape_html' => 'markup/render.php',
     'phutil_escape_html_newlines' => 'markup/render.php',
@@ -564,6 +565,7 @@
     'phutil_implode_html' => 'markup/render.php',
     'phutil_ini_decode' => 'utils/utils.php',
     'phutil_is_hiphop_runtime' => 'utils/utils.php',
+    'phutil_is_natural_list' => 'utils/utils.php',
     'phutil_is_system_locale_available' => 'utils/utf8.php',
     'phutil_is_utf8' => 'utils/utf8.php',
     'phutil_is_utf8_slowly' => 'utils/utf8.php',
diff --git a/src/conduit/ConduitClient.php b/src/conduit/ConduitClient.php
--- a/src/conduit/ConduitClient.php
+++ b/src/conduit/ConduitClient.php
@@ -348,7 +348,7 @@
     $out = array();
 
     if (is_array($data)) {
-      if (!$data || (array_keys($data) == range(0, count($data) - 1))) {
+      if (phutil_is_natural_list($data)) {
         $out[] = 'A';
         $out[] = count($data);
         $out[] = ':';
diff --git a/src/moduleutils/PhutilBootloader.php b/src/moduleutils/PhutilBootloader.php
--- a/src/moduleutils/PhutilBootloader.php
+++ b/src/moduleutils/PhutilBootloader.php
@@ -62,6 +62,15 @@
 
     $this->registeredLibraries[$name] = $path;
 
+    // If we're loading libphutil itself, load the utility functions first so
+    // we can safely call functions like "id()" when handling errors. In
+    // particular, this improves error behavior when "utils.php" itself can
+    // not load.
+    if ($name === 'phutil') {
+      $root = $this->getLibraryRoot('phutil');
+      $this->executeInclude($root.'/utils/utils.php');
+    }
+
     // For libphutil v2 libraries, load all functions when we load the library.
 
     if (!class_exists('PhutilSymbolLoader', false)) {
diff --git a/src/parser/PhutilJSON.php b/src/parser/PhutilJSON.php
--- a/src/parser/PhutilJSON.php
+++ b/src/parser/PhutilJSON.php
@@ -118,7 +118,7 @@
    */
   private function encodeFormattedValue($value, $depth) {
     if (is_array($value)) {
-      if (empty($value) || array_keys($value) === range(0, count($value) - 1)) {
+      if (phutil_is_natural_list($value)) {
         return $this->encodeFormattedArray($value, $depth);
       } else {
         return $this->encodeFormattedObject($value, $depth);
diff --git a/src/parser/PhutilTypeSpec.php b/src/parser/PhutilTypeSpec.php
--- a/src/parser/PhutilTypeSpec.php
+++ b/src/parser/PhutilTypeSpec.php
@@ -93,7 +93,7 @@
         if (!is_array($value)) {
           throw new PhutilTypeCheckException($this, $value, $name);
         }
-        if ($value && (array_keys($value) !== range(0, count($value) - 1))) {
+        if ($value && !phutil_is_natural_list($value)) {
           throw new PhutilTypeCheckException($this, $value, $name);
         }
         try {
@@ -209,7 +209,7 @@
       return get_class($value);
     } else if (is_array($value)) {
       $vtype = self::getTypeOfVector($value);
-      if ($value && (array_keys($value) === range(0, count($value) - 1))) {
+      if ($value && phutil_is_natural_list($value)) {
         return 'list<'.$vtype.'>';
       } else {
         $ktype = self::getTypeOfVector(array_keys($value));
diff --git a/src/symbols/PhutilSymbolLoader.php b/src/symbols/PhutilSymbolLoader.php
--- a/src/symbols/PhutilSymbolLoader.php
+++ b/src/symbols/PhutilSymbolLoader.php
@@ -386,11 +386,11 @@
     $load_failed = null;
     if ($is_function) {
       if (!function_exists($name)) {
-        $load_failed = pht('function');
+        $load_failed = 'function';
       }
     } else {
       if (!class_exists($name, false) && !interface_exists($name, false)) {
-        $load_failed = pht('class or interface');
+        $load_failed = 'class/interface';
       }
     }
 
@@ -400,14 +400,13 @@
         $name,
         $load_failed,
         pht(
-          "the symbol map for library '%s' (at '%s') claims this %s is ".
-          "defined in '%s', but loading that source file did not cause the ".
-          "%s to become defined.",
+          'The symbol map for library "%s" (at "%s") claims this symbol '.
+          '(of type "%s") is defined in "%s", but loading that source file '.
+          'did not cause the symbol to become defined.',
           $lib_name,
           $lib_path,
           $load_failed,
-          $where,
-          $load_failed));
+          $where));
     }
   }
 
diff --git a/src/symbols/exception/PhutilMissingSymbolException.php b/src/symbols/exception/PhutilMissingSymbolException.php
--- a/src/symbols/exception/PhutilMissingSymbolException.php
+++ b/src/symbols/exception/PhutilMissingSymbolException.php
@@ -5,22 +5,24 @@
   public function __construct($symbol, $type, $reason) {
     parent::__construct(
       pht(
-        "Failed to load %s '%s': %s\n\n".
-        "If you are not a developer, this almost always means that a library ".
-        "is out of date. For example, you may have upgraded `phabricator` ".
-        "without upgrading `libphutil`, or vice versa. It might also mean ".
-        "that you need to restart Apache or PHP-FPM. Make sure all libraries ".
-        "are up to date and all services have been restarted.\n\n".
-        "If you are a developer and this symbol was recently added or moved, ".
-        "your library map may need to be rebuilt. You can rebuild the map by ".
-        "running '%s'. For more information, see:\n\n".
-        "%s",
+        'Failed to load %s "%s".'.
+        "\n\n".
+        '%s'.
+        "\n\n".
+        'If you are not a developer, this almost always means that a library '.
+        'is out of date. For example, you may have upgraded "phabricator/" '.
+        'without upgrading "libphutil/", or vice versa. It might also mean '.
+        'that you need to restart Apache or PHP-FPM. Make sure all libraries '.
+        'are up to date and all services have been restarted.'.
+        "\n\n".
+        'If you are a developer and this symbol was recently added or '.
+        'moved, your library map may need to be rebuilt. You can rebuild '.
+        'the map by running "arc liberate".'.
+        "\n\n".
+        'For more information, see: https://phurl.io/newclasses',
         $type,
         $symbol,
-        $reason,
-        'arc liberate',
-        'https://secure.phabricator.com/book/phabcontrib/article/'.
-          'adding_new_classes/'));
+        $reason));
   }
 
 }
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
@@ -918,4 +918,21 @@
     }
   }
 
+  public function testNaturalList() {
+    $cases = array(
+      array(true, array()),
+      array(true, array(0 => true, 1 => true, 2 => true)),
+      array(true, array('a', 'b', 'c')),
+      array(false, array(0 => true, 2 => true, 1 => true)),
+      array(false, array(1 => true)),
+      array(false, array('sound' => 'quack')),
+    );
+
+    foreach ($cases as $case) {
+      list($expect, $value) = $case;
+      $this->assertEqual($expect, phutil_is_natural_list($value));
+    }
+  }
+
+
 }
diff --git a/src/utils/utils.php b/src/utils/utils.php
--- a/src/utils/utils.php
+++ b/src/utils/utils.php
@@ -1476,7 +1476,7 @@
     }
 
     // Don't show keys for non-associative arrays.
-    $show_keys = (array_keys($var) !== range(0, count($var) - 1));
+    $show_keys = !phutil_is_natural_list($var);
 
     $output = array();
     $output[] = 'array(';
@@ -1757,3 +1757,40 @@
 
   return (string)$value;
 }
+
+
+/**
+ * Return a short, human-readable description of an object's type.
+ *
+ * This is mostly useful for raising errors like "expected x() to return a Y,
+ * but it returned a Z".
+ *
+ * This is similar to "get_type()", but describes objects and arrays in more
+ * detail.
+ *
+ * @param wild Anything.
+ * @return string Human-readable description of the value's type.
+ */
+function phutil_describe_type($value) {
+  return PhutilTypeSpec::getTypeOf($value);
+}
+
+
+/**
+ * Test if a list has the natural numbers (1, 2, 3, and so on) as keys, in
+ * order.
+ *
+ * @return bool True if the list is a natural list.
+ */
+function phutil_is_natural_list(array $list) {
+  $expect = 0;
+
+  foreach ($list as $key => $item) {
+    if ($key !== $expect) {
+      return false;
+    }
+    $expect++;
+  }
+
+  return true;
+}