diff --git a/NEWS b/NEWS
index 2b2a06e3..f5943431 100644
--- a/NEWS
+++ b/NEWS
@@ -22,6 +22,7 @@ NEWS ( CHANGELOG and HISTORY )                                     HTMLPurifier
 . Refactored unit tests to minimize duplication
 . XSS attack sheet updated
 . configdoc.xml now has xml:space attached to default value nodes
+. Allow configuration directives to permit null values
 
 1.1.3, unknown projected release date
 (bugfix release, may be dropped if no major bugs are found before features)
diff --git a/configdoc/generate.php b/configdoc/generate.php
index acb059ef..93328356 100644
--- a/configdoc/generate.php
+++ b/configdoc/generate.php
@@ -110,9 +110,12 @@ foreach($schema->info as $namespace_name => $namespace_info) {
         $dom_constraints = $dom_document->createElement('constraints');
         $dom_directive->appendChild($dom_constraints);
         
-        $dom_constraints->appendChild(
-            $dom_document->createElement('type', $info->type)
-        );
+        $dom_type = $dom_document->createElement('type', $info->type);
+        if ($info->allow_null) {
+            $dom_type->setAttribute('allow-null', 'yes');
+        }
+        $dom_constraints->appendChild($dom_type);
+        
         if ($info->allowed !== true) {
             $dom_allowed = $dom_document->createElement('allowed');
             $dom_constraints->appendChild($dom_allowed);
@@ -128,6 +131,8 @@ foreach($schema->info as $namespace_name => $namespace_info) {
             $default = $raw_default ? 'true' : 'false';
         } elseif (is_string($raw_default)) {
             $default = "\"$raw_default\"";
+        } elseif (is_null($raw_default)) {
+            $default = 'null';
         } else {
             $default = print_r(
                     $schema->defaults[$namespace_name][$name], true
diff --git a/configdoc/styles/plain.xsl b/configdoc/styles/plain.xsl
index f97970c0..13f57352 100644
--- a/configdoc/styles/plain.xsl
+++ b/configdoc/styles/plain.xsl
@@ -69,7 +69,7 @@
         <xsl:apply-templates />
     </xsl:template>
     <xsl:template match="directive/name">
-        <h3 id="{../@id}"><xsl:value-of select="." /></h3>
+        <h3 id="{../@id}"><xsl:value-of select="../@id" /></h3>
     </xsl:template>
     <xsl:template match="directive/constraints">
         <table class="constraints">
@@ -99,6 +99,9 @@
                 <xsl:variable name="type" select="text()" />
                 <xsl:attribute name="class">type type-<xsl:value-of select="$type" /></xsl:attribute>
                 <xsl:value-of select="$typeLookup/types/type[@id=$type]/text()" />
+                <xsl:if test="@allow-null='yes'">
+                    (or null)
+                </xsl:if>
             </td>
         </tr>
     </xsl:template>
diff --git a/library/HTMLPurifier/Config.php b/library/HTMLPurifier/Config.php
index 18631cc8..b2ac0e70 100644
--- a/library/HTMLPurifier/Config.php
+++ b/library/HTMLPurifier/Config.php
@@ -80,8 +80,11 @@ class HTMLPurifier_Config
                 E_USER_WARNING);
             return;
         }
-        $value = $this->def->validate($value,
-                                      $this->def->info[$namespace][$key]->type);
+        $value = $this->def->validate(
+                    $value,
+                    $this->def->info[$namespace][$key]->type,
+                    $this->def->info[$namespace][$key]->allow_null
+                 );
         if (is_string($value)) {
             // resolve value alias if defined
             if (isset($this->def->info[$namespace][$key]->aliases[$value])) {
@@ -95,7 +98,7 @@ class HTMLPurifier_Config
                 }
             }
         }
-        if ($value === null) {
+        if ($this->def->isError($value)) {
             trigger_error('Value is of invalid type', E_USER_WARNING);
             return;
         }
diff --git a/library/HTMLPurifier/ConfigSchema.php b/library/HTMLPurifier/ConfigSchema.php
index 99806bd9..79633b20 100644
--- a/library/HTMLPurifier/ConfigSchema.php
+++ b/library/HTMLPurifier/ConfigSchema.php
@@ -1,5 +1,7 @@
 <?php
 
+require_once 'HTMLPurifier/Error.php';
+
 /**
  * Configuration definition, defines directives and their defaults.
  * @todo The ability to define things multiple times is confusing and should
@@ -111,12 +113,19 @@ class HTMLPurifier_ConfigSchema {
                 return;
             }
         } else {
+            // process modifiers
+            $type_values = explode('/', $type, 2);
+            $type = $type_values[0];
+            $modifier = isset($type_values[1]) ? $type_values[1] : false;
+            $allow_null = ($modifier === 'null');
+            
             if (!isset($def->types[$type])) {
                 trigger_error('Invalid type for configuration directive',
                     E_USER_ERROR);
                 return;
             }
-            if ($def->validate($default, $type) === null) {
+            $default = $def->validate($default, $type, $allow_null);
+            if ($def->isError($default)) {
                 trigger_error('Default value does not match directive type',
                     E_USER_ERROR);
                 return;
@@ -124,6 +133,7 @@ class HTMLPurifier_ConfigSchema {
             $def->info[$namespace][$name] =
                 new HTMLPurifier_ConfigEntity_Directive();
             $def->info[$namespace][$name]->type = $type;
+            $def->info[$namespace][$name]->allow_null = $allow_null;
             $def->defaults[$namespace][$name]   = $default;
         }
         $backtrace = debug_backtrace();
@@ -212,36 +222,37 @@ class HTMLPurifier_ConfigSchema {
     /**
      * Validate a variable according to type. Return null if invalid.
      */
-    function validate($var, $type) {
+    function validate($var, $type, $allow_null = false) {
         if (!isset($this->types[$type])) {
             trigger_error('Invalid type', E_USER_ERROR);
             return;
         }
+        if ($allow_null && $var === null) return null;
         switch ($type) {
             case 'mixed':
                 return $var;
             case 'istring':
             case 'string':
-                if (!is_string($var)) return;
+                if (!is_string($var)) break;
                 if ($type === 'istring') $var = strtolower($var);
                 return $var;
             case 'int':
                 if (is_string($var) && ctype_digit($var)) $var = (int) $var;
-                elseif (!is_int($var)) return;
+                elseif (!is_int($var)) break;
                 return $var;
             case 'float':
                 if (is_string($var) && is_numeric($var)) $var = (float) $var;
-                elseif (!is_float($var)) return;
+                elseif (!is_float($var)) break;
                 return $var;
             case 'bool':
                 if (is_int($var) && ($var === 0 || $var === 1)) {
                     $var = (bool) $var;
-                } elseif (!is_bool($var)) return;
+                } elseif (!is_bool($var)) break;
                 return $var;
             case 'list':
             case 'hash':
             case 'lookup':
-                if (!is_array($var)) return;
+                if (!is_array($var)) break;
                 $keys = array_keys($var);
                 if ($keys === array_keys($keys)) {
                     if ($type == 'list') return $var;
@@ -251,7 +262,7 @@ class HTMLPurifier_ConfigSchema {
                             $new[$key] = true;
                         }
                         return $new;
-                    } else return;
+                    } else break;
                 }
                 if ($type === 'lookup') {
                     foreach ($var as $key => $value) {
@@ -260,8 +271,13 @@ class HTMLPurifier_ConfigSchema {
                 }
                 return $var;
         }
+        $error = new HTMLPurifier_Error();
+        return $error;
     }
     
+    /**
+     * Takes an absolute path and munges it into a more manageable relative path
+     */
     function mungeFilename($filename) {
         $offset = strrpos($filename, 'HTMLPurifier');
         $filename = substr($filename, $offset);
@@ -269,6 +285,14 @@ class HTMLPurifier_ConfigSchema {
         return $filename;
     }
     
+    /**
+     * Checks if var is an HTMLPurifier_Error object
+     */
+    function isError($var) {
+        if (!is_object($var)) return false;
+        if (!is_a($var, 'HTMLPurifier_Error')) return false;
+        return true;
+    }
 }
 
 /**
@@ -318,6 +342,13 @@ class HTMLPurifier_ConfigEntity_Directive extends HTMLPurifier_ConfigEntity
      *      - mixed (anything goes)
      */
     var $type = 'mixed';
+    
+    /**
+     * Is null allowed? Has no affect for mixed type.
+     * @bool
+     */
+    var $allow_null = false;
+    
     /**
      * Plaintext descriptions of the configuration entity is. Organized by
      * file and line number, so multiple descriptions are allowed.
diff --git a/library/HTMLPurifier/Error.php b/library/HTMLPurifier/Error.php
new file mode 100644
index 00000000..adc81dc5
--- /dev/null
+++ b/library/HTMLPurifier/Error.php
@@ -0,0 +1,8 @@
+<?php
+
+/**
+ * Return object from functions that signifies error when null doesn't cut it
+ */
+class HTMLPurifier_Error {}
+
+?>
\ No newline at end of file
diff --git a/tests/HTMLPurifier/ConfigSchemaTest.php b/tests/HTMLPurifier/ConfigSchemaTest.php
index fe04f053..364caa91 100644
--- a/tests/HTMLPurifier/ConfigSchemaTest.php
+++ b/tests/HTMLPurifier/ConfigSchemaTest.php
@@ -231,6 +231,17 @@ class HTMLPurifier_ConfigSchemaTest extends UnitTestCase
         $this->swallowErrors();
         
         
+        
+        // define a directive that allows null
+        HTMLPurifier_ConfigSchema::define(
+            'Core', 'Foobaz', null, 'string/null',
+            'Nulls are allowed if you add on /null, cool huh?'
+        );
+        
+        $this->assertNoErrors();
+        $this->swallowErrors();
+        
+        
         // define a directive with bad characters
         HTMLPurifier_ConfigSchema::define(
             'Core', 'Core.Attr', 10, 'int',
@@ -258,7 +269,11 @@ class HTMLPurifier_ConfigSchemaTest extends UnitTestCase
     }
     
     function assertInvalid($var, $type) {
-        $this->assertIdentical($this->our_copy->validate($var, $type), null);
+        $this->assertTrue(
+            $this->our_copy->isError(
+                $this->our_copy->validate($var, $type)
+            )
+        );
     }
     
     function testValidate() {
@@ -271,6 +286,7 @@ class HTMLPurifier_ConfigSchemaTest extends UnitTestCase
         $this->assertValid(0, 'bool', false);
         $this->assertValid(1, 'bool', true);
         $this->assertInvalid(34, 'bool');
+        $this->assertInvalid(null, 'bool');
         $this->assertValid(array('1', '2', '3'), 'list');
         $this->assertValid(array('1' => true, '2' => true), 'lookup');
         $this->assertValid(array('1', '2'), 'lookup', array('1' => true, '2' => true));
@@ -281,6 +297,22 @@ class HTMLPurifier_ConfigSchemaTest extends UnitTestCase
         
     }
     
+    function testValidate_null() {
+        
+        $this->assertTrue(
+            $this->our_copy->isError(
+                $this->our_copy->validate(null, 'string', false)
+            )
+        );
+        
+        $this->assertFalse(
+            $this->our_copy->isError(
+                $this->our_copy->validate(null, 'string', true)
+            )
+        );
+        
+    }
+    
     function assertMungeFilename($oldname, $newname) {
         $this->assertIdentical(
             $this->our_copy->mungeFilename($oldname),