diff --git a/src/FilterRule/StripHtml.php b/src/FilterRule/StripHtml.php
index 85efc14..6ebd99b 100644
--- a/src/FilterRule/StripHtml.php
+++ b/src/FilterRule/StripHtml.php
@@ -40,6 +40,10 @@ public function __construct($excludeTags = null)
      */
     public function filter($value)
     {
+        if ($value === null) {
+            return $value;
+        }
+
         return strip_tags($value, $this->excludeTags);
     }
 }
diff --git a/src/FilterRule/Trim.php b/src/FilterRule/Trim.php
index 8c20fc3..36dcc1e 100644
--- a/src/FilterRule/Trim.php
+++ b/src/FilterRule/Trim.php
@@ -40,6 +40,10 @@ public function __construct($characters = null)
      */
     public function filter($value)
     {
+        if ($value === null) {
+            return $value;
+        }
+
         if ($this->characters === null) {
             return trim($value);
         }
diff --git a/tests/FilterRule/StripHtmlTest.php b/tests/FilterRule/StripHtmlTest.php
index 8a7e62a..904097f 100644
--- a/tests/FilterRule/StripHtmlTest.php
+++ b/tests/FilterRule/StripHtmlTest.php
@@ -32,7 +32,7 @@ public function testStripHtmlFilterRule($value, $excludeTags, $filteredValue)
             'test' => $value
         ]);
 
-        $this->assertEquals($filteredValue, $result['test']);
+        $this->assertSame($filteredValue, $result['test']);
     }
 
     /**
@@ -44,6 +44,7 @@ public function getStripHtmlResults()
             ['', '', ''],
             ['<p><strong>t</strong>ext</p>', null, 'text'],
             ['<p><strong>t</strong>ext</p>', '<p>', '<p>text</p>'],
+            [null, null, null],
         ];
     }
 }
diff --git a/tests/FilterRule/TrimTest.php b/tests/FilterRule/TrimTest.php
index eaedf07..ab6760f 100644
--- a/tests/FilterRule/TrimTest.php
+++ b/tests/FilterRule/TrimTest.php
@@ -36,7 +36,7 @@ public function testTrimFilterRule($value, $filteredValue, $characters)
             'test' => $value
         ]);
 
-        $this->assertEquals($filteredValue, $result['test']);
+        $this->assertSame($filteredValue, $result['test']);
     }
 
     /**
@@ -54,6 +54,7 @@ public function getTrimResults()
             ['   abc      ', 'abc', null],
             ['  !~! ', '!~!', null],
             ['	 tabtab 	', ' tabtab ', "\t"],
+            [null, null, null],
         ];
     }
 }