diff --git a/library/HTMLPurifier/ConfigSchema/Validator.php b/library/HTMLPurifier/ConfigSchema/Validator.php
index ff10a93a..48c0f82a 100644
--- a/library/HTMLPurifier/ConfigSchema/Validator.php
+++ b/library/HTMLPurifier/ConfigSchema/Validator.php
@@ -30,7 +30,6 @@ class HTMLPurifier_ConfigSchema_Validator
     public function validateNamespace($n) {
         $this->context = "namespace '{$n->namespace}'";
         $this->with($n, 'namespace')
-            ->assertIsString()
             ->assertNotEmpty()
             ->assertAlnum();
         $this->with($n, 'description')
@@ -40,12 +39,27 @@ class HTMLPurifier_ConfigSchema_Validator
     
     public function validateDirective($d) {
         $this->context = "directive '{$d->id}'";
+        $this->validateId($d->id);
+    }
+    
+    public function validateId($id) {
+        $this->context = "id '$id'";
+        $this->with($id, 'namespace')
+            ->assertNotEmpty()
+            ->assertAlnum();
+        $this->with($id, 'directive')
+            ->assertNotEmpty()
+            ->assertAlnum();
     }
     
     // protected helper functions
     
     protected function with($obj, $member) {
-        return new HTMLPurifier_ConfigSchema_ValidatorAtom($this->interchange, $this->context, $obj, $member);
+        return new HTMLPurifier_ConfigSchema_ValidatorAtom($this->context, $obj, $member);
+    }
+    
+    protected function error($msg) {
+        throw new HTMLPurifier_ConfigSchema_Exception($msg . ' in ' . $this->context);
     }
     
 }
diff --git a/library/HTMLPurifier/ConfigSchema/ValidatorAtom.php b/library/HTMLPurifier/ConfigSchema/ValidatorAtom.php
index 0ebacf80..b799d67b 100644
--- a/library/HTMLPurifier/ConfigSchema/ValidatorAtom.php
+++ b/library/HTMLPurifier/ConfigSchema/ValidatorAtom.php
@@ -3,15 +3,15 @@
 /**
  * Fluent interface for validating the contents of member variables.
  * This should be immutable. See HTMLPurifier_ConfigSchema_Validator for
- * use-cases.
+ * use-cases. We name this an 'atom' because it's ONLY for validations that
+ * are independent and usually scalar.
  */
 class HTMLPurifier_ConfigSchema_ValidatorAtom
 {
     
-    protected $interchange, $context, $obj, $member, $contents;
+    protected $context, $obj, $member, $contents;
     
-    public function __construct($interchange, $context, $obj, $member) {
-        $this->interchange = $interchange;
+    public function __construct($context, $obj, $member) {
         $this->context     = $context;
         $this->obj         = $obj;
         $this->member      = $member;
@@ -24,11 +24,12 @@ class HTMLPurifier_ConfigSchema_ValidatorAtom
     }
     
     public function assertNotNull() {
-        if (is_null($this->contents)) $this->error('must not be null');
+        if ($this->contents === null) $this->error('must not be null');
         return $this;
     }
     
     public function assertAlnum() {
+        $this->assertIsString();
         if (!ctype_alnum($this->contents)) $this->error('must be alphanumeric');
         return $this;
     }
diff --git a/tests/HTMLPurifier/ConfigSchema/ValidatorAtomTest.php b/tests/HTMLPurifier/ConfigSchema/ValidatorAtomTest.php
new file mode 100644
index 00000000..7dc027f9
--- /dev/null
+++ b/tests/HTMLPurifier/ConfigSchema/ValidatorAtomTest.php
@@ -0,0 +1,58 @@
+<?php
+
+class HTMLPurifier_ConfigSchema_ValidatorAtomTest extends UnitTestCase
+{
+    
+    protected function expectValidationException($msg) {
+        $this->expectException(new HTMLPurifier_ConfigSchema_Exception($msg));
+    }
+    
+    public function makeAtom($value) {
+        $obj = new stdClass();
+        $obj->property = $value;
+        // Note that 'property' and 'context' are magic wildcard values
+        return new HTMLPurifier_ConfigSchema_ValidatorAtom('context', $obj, 'property');
+    }
+    
+    public function testAssertIsString() {
+        $this->makeAtom('foo')->assertIsString();
+    }
+    
+    public function testAssertIsStringFail() {
+        $this->expectValidationException("Member variable 'property' in context must be a string");
+        $this->makeAtom(3)->assertIsString();
+    }
+    
+    public function testAssertNotNull() {
+        $this->makeAtom('foo')->assertNotNull();
+    }
+    
+    public function testAssertNotNullFail() {
+        $this->expectValidationException("Member variable 'property' in context must not be null");
+        $this->makeAtom(null)->assertNotNull();
+    }
+    
+    public function testAssertAlnum() {
+        $this->makeAtom('foo2')->assertAlnum();
+    }
+    
+    public function testAssertAlnumFail() {
+        $this->expectValidationException("Member variable 'property' in context must be alphanumeric");
+        $this->makeAtom('%a')->assertAlnum();
+    }
+    
+    public function testAssertAlnumFailIsString() {
+        $this->expectValidationException("Member variable 'property' in context must be a string");
+        $this->makeAtom(3)->assertAlnum();
+    }
+    
+    public function testAssertNotEmpty() {
+        $this->makeAtom('foo')->assertNotEmpty();
+    }
+    
+    public function testAssertNotEmptyFail() {
+        $this->expectValidationException("Member variable 'property' in context must not be empty");
+        $this->makeAtom('')->assertNotEmpty();
+    }
+    
+}
diff --git a/tests/HTMLPurifier/ConfigSchema/ValidatorTest.php b/tests/HTMLPurifier/ConfigSchema/ValidatorTest.php
index 1bb5246d..11404b38 100644
--- a/tests/HTMLPurifier/ConfigSchema/ValidatorTest.php
+++ b/tests/HTMLPurifier/ConfigSchema/ValidatorTest.php
@@ -18,6 +18,20 @@ class HTMLPurifier_ConfigSchema_ValidatorTest extends UnitTestCase
         $obj->namespace   = $namespace;
         $obj->description = $description;
         $this->interchange->addNamespace($obj);
+        return $obj;
+    }
+    
+    protected function addDirective($namespace, $directive, $type = 'string', $description = 'Description') {
+        $obj = new HTMLPurifier_ConfigSchema_Interchange_Directive();
+        $obj->id = $this->makeId($namespace, $directive);
+        $obj->type = $type;
+        $obj->description = $description;
+        $this->interchange->addDirective($obj);
+        return $obj; // for future editing
+    }
+    
+    protected function makeId($namespace, $directive) {
+        return new HTMLPurifier_ConfigSchema_Interchange_Id($namespace, $directive);
     }
     
     /**
@@ -36,12 +50,6 @@ class HTMLPurifier_ConfigSchema_ValidatorTest extends UnitTestCase
         $this->validate();
     }
     
-    public function testNamespaceNamespaceString() {
-        $this->addNamespace(3, 'Description');
-        $this->expectValidationException("Member variable 'namespace' in namespace '3' must be a string");
-        $this->validate();
-    }
-    
     public function testNamespaceNamespaceNotEmpty() {
         $this->addNamespace('0', 'Description');
         $this->expectValidationException("Member variable 'namespace' in namespace '0' must not be empty");
@@ -54,16 +62,14 @@ class HTMLPurifier_ConfigSchema_ValidatorTest extends UnitTestCase
         $this->validate();
     }
     
-    public function testNamespaceDescriptionString() {
-        $this->addNamespace('Ns', 3);
-        $this->expectValidationException("Member variable 'description' in namespace 'Ns' must be a string");
-        $this->validate();
-    }
-    
     public function testNamespaceDescriptionNotEmpty() {
         $this->addNamespace('Ns', '');
         $this->expectValidationException("Member variable 'description' in namespace 'Ns' must not be empty");
         $this->validate();
     }
     
+    public function testDirectiveIdNamespaceNotEmpty() {
+        $this->addDirective('', 'Dir');
+    }
+    
 }