diff --git a/cache/stores/session/lib.php b/cache/stores/session/lib.php
index b2f77780b13..18e0ccfa73c 100644
--- a/cache/stores/session/lib.php
+++ b/cache/stores/session/lib.php
@@ -218,11 +218,10 @@ class cachestore_session extends session_data_store implements cache_is_key_awar
      * @return mixed The data that was associated with the key, or false if the key did not exist.
      */
     public function get($key) {
-        $maxtime = cache::now() - $this->ttl;
-        if (array_key_exists($key, $this->store)) {
+        if (isset($this->store[$key])) {
             if ($this->ttl == 0) {
-                return $this->store[$key];
-            } else if ($this->store[$key][1] >= $maxtime) {
+                return $this->store[$key][0];
+            } else if ($this->store[$key][1] >= (cache::now() - $this->ttl)) {
                 return $this->store[$key][0];
             }
         }
@@ -240,12 +239,15 @@ class cachestore_session extends session_data_store implements cache_is_key_awar
      */
     public function get_many($keys) {
         $return = array();
-        $maxtime = cache::now() - $this->ttl;
+        if ($this->ttl != 0) {
+            $maxtime = cache::now() - $this->ttl;
+        }
+
         foreach ($keys as $key) {
             $return[$key] = false;
-            if (array_key_exists($key, $this->store)) {
+            if (isset($this->store[$key])) {
                 if ($this->ttl == 0) {
-                    $return[$key] = $this->store[$key];
+                    $return[$key] = $this->store[$key][0];
                 } else if ($this->store[$key][1] >= $maxtime) {
                     $return[$key] = $this->store[$key][0];
                 }
@@ -263,7 +265,7 @@ class cachestore_session extends session_data_store implements cache_is_key_awar
      */
     public function set($key, $data) {
         if ($this->ttl == 0) {
-            $this->store[$key] = $data;
+            $this->store[$key][0] = $data;
         } else {
             $this->store[$key] = array($data, cache::now());
         }
@@ -294,11 +296,10 @@ class cachestore_session extends session_data_store implements cache_is_key_awar
      * @return bool
      */
     public function has($key) {
-        $maxtime = cache::now() - $this->ttl;
-        if (array_key_exists($key, $this->store)) {
+        if (isset($this->store[$key])) {
             if ($this->ttl == 0) {
                 return true;
-            } else if ($this->store[$key][1] >= $maxtime) {
+            } else if ($this->store[$key][1] >= (cache::now() - $this->ttl)) {
                 return true;
             }
         }
@@ -312,9 +313,12 @@ class cachestore_session extends session_data_store implements cache_is_key_awar
      * @return bool
      */
     public function has_all(array $keys) {
-        $maxtime = cache::now() - $this->ttl;
+        if ($this->ttl != 0) {
+            $maxtime = cache::now() - $this->ttl;
+        }
+
         foreach ($keys as $key) {
-            if (!array_key_exists($key, $this->store)) {
+            if (!isset($this->store[$key])) {
                 return false;
             }
             if ($this->ttl != 0 && $this->store[$key][1] < $maxtime) {
@@ -331,9 +335,12 @@ class cachestore_session extends session_data_store implements cache_is_key_awar
      * @return bool
      */
     public function has_any(array $keys) {
-        $maxtime = cache::now() - $this->ttl;
+        if ($this->ttl != 0) {
+            $maxtime = cache::now() - $this->ttl;
+        }
+
         foreach ($keys as $key) {
-            if (array_key_exists($key, $this->store) && ($this->ttl == 0 || $this->store[$key][1] >= $maxtime)) {
+            if (isset($this->store[$key]) && ($this->ttl == 0 || $this->store[$key][1] >= $maxtime)) {
                 return true;
             }
         }
@@ -439,4 +446,4 @@ class cachestore_session extends session_data_store implements cache_is_key_awar
         }
         return $return;
     }
-}
\ No newline at end of file
+}
diff --git a/cache/stores/static/lib.php b/cache/stores/static/lib.php
index 405474abc4c..5c38bc8c719 100644
--- a/cache/stores/static/lib.php
+++ b/cache/stores/static/lib.php
@@ -213,11 +213,10 @@ class cachestore_static extends static_data_store implements cache_is_key_aware
      * @return mixed The data that was associated with the key, or false if the key did not exist.
      */
     public function get($key) {
-        $maxtime = cache::now() - $this->ttl;
-        if (array_key_exists($key, $this->store)) {
+        if (isset($this->store[$key])) {
             if ($this->ttl == 0) {
-                return $this->store[$key];
-            } else if ($this->store[$key][1] >= $maxtime) {
+                return $this->store[$key][0];
+            } else if ($this->store[$key][1] >= (cache::now() - $this->ttl)) {
                 return $this->store[$key][0];
             }
         }
@@ -235,12 +234,15 @@ class cachestore_static extends static_data_store implements cache_is_key_aware
      */
     public function get_many($keys) {
         $return = array();
-        $maxtime = cache::now() - $this->ttl;
+        if ($this->ttl != 0) {
+            $maxtime = cache::now() - $this->ttl;
+        }
+
         foreach ($keys as $key) {
             $return[$key] = false;
-            if (array_key_exists($key, $this->store)) {
+            if (isset($this->store[$key])) {
                 if ($this->ttl == 0) {
-                    $return[$key] = $this->store[$key];
+                    $return[$key] = $this->store[$key][0];
                 } else if ($this->store[$key][1] >= $maxtime) {
                     $return[$key] = $this->store[$key][0];
                 }
@@ -258,7 +260,7 @@ class cachestore_static extends static_data_store implements cache_is_key_aware
      */
     public function set($key, $data) {
         if ($this->ttl == 0) {
-            $this->store[$key] = $data;
+            $this->store[$key][0] = $data;
         } else {
             $this->store[$key] = array($data, cache::now());
         }
@@ -289,11 +291,10 @@ class cachestore_static extends static_data_store implements cache_is_key_aware
      * @return bool
      */
     public function has($key) {
-        $maxtime = cache::now() - $this->ttl;
-        if (array_key_exists($key, $this->store)) {
+        if (isset($this->store[$key])) {
             if ($this->ttl == 0) {
                 return true;
-            } else if ($this->store[$key][1] >= $maxtime) {
+            } else if ($this->store[$key][1] >= (cache::now() - $this->ttl)) {
                 return true;
             }
         }
@@ -307,9 +308,12 @@ class cachestore_static extends static_data_store implements cache_is_key_aware
      * @return bool
      */
     public function has_all(array $keys) {
-        $maxtime = cache::now() - $this->ttl;
+        if ($this->ttl != 0) {
+            $maxtime = cache::now() - $this->ttl;
+        }
+
         foreach ($keys as $key) {
-            if (!array_key_exists($key, $this->store)) {
+            if (!isset($this->store[$key])) {
                 return false;
             }
             if ($this->ttl != 0 && $this->store[$key][1] < $maxtime) {
@@ -326,9 +330,12 @@ class cachestore_static extends static_data_store implements cache_is_key_aware
      * @return bool
      */
     public function has_any(array $keys) {
-        $maxtime = cache::now() - $this->ttl;
+        if ($this->ttl != 0) {
+            $maxtime = cache::now() - $this->ttl;
+        }
+
         foreach ($keys as $key) {
-            if (array_key_exists($key, $this->store) && ($this->ttl == 0 || $this->store[$key][1] >= $maxtime)) {
+            if (isset($this->store[$key]) && ($this->ttl == 0 || $this->store[$key][1] >= $maxtime)) {
                 return true;
             }
         }
@@ -411,4 +418,4 @@ class cachestore_static extends static_data_store implements cache_is_key_aware
     public function my_name() {
         return $this->name;
     }
-}
\ No newline at end of file
+}
diff --git a/cache/tests/cache_test.php b/cache/tests/cache_test.php
index 5293eeccc73..bc7713531f9 100644
--- a/cache/tests/cache_test.php
+++ b/cache/tests/cache_test.php
@@ -192,41 +192,43 @@ class cache_phpunit_tests extends advanced_testcase {
      */
     protected function run_on_cache(cache_loader $cache) {
         $key = 'testkey';
-        $datascalar = 'test data';
+        $datascalars = array('test data', null);
         $dataarray = array('test' => 'data', 'part' => 'two');
         $dataobject = (object)$dataarray;
 
-        $this->assertTrue($cache->purge());
+        foreach ($datascalars as $datascalar) {
+            $this->assertTrue($cache->purge());
 
-        // Check all read methods.
-        $this->assertFalse($cache->get($key));
-        $this->assertFalse($cache->has($key));
-        $result = $cache->get_many(array($key));
-        $this->assertCount(1, $result);
-        $this->assertFalse(reset($result));
-        $this->assertFalse($cache->has_any(array($key)));
-        $this->assertFalse($cache->has_all(array($key)));
+            // Check all read methods.
+            $this->assertFalse($cache->get($key));
+            $this->assertFalse($cache->has($key));
+            $result = $cache->get_many(array($key));
+            $this->assertCount(1, $result);
+            $this->assertFalse(reset($result));
+            $this->assertFalse($cache->has_any(array($key)));
+            $this->assertFalse($cache->has_all(array($key)));
 
-        // Set the data.
-        $this->assertTrue($cache->set($key, $datascalar));
-        // Setting it more than once should be permitted.
-        $this->assertTrue($cache->set($key, $datascalar));
+            // Set the data.
+            $this->assertTrue($cache->set($key, $datascalar));
+            // Setting it more than once should be permitted.
+            $this->assertTrue($cache->set($key, $datascalar));
 
-        // Recheck the read methods.
-        $this->assertEquals($datascalar, $cache->get($key));
-        $this->assertTrue($cache->has($key));
-        $result = $cache->get_many(array($key));
-        $this->assertCount(1, $result);
-        $this->assertEquals($datascalar, reset($result));
-        $this->assertTrue($cache->has_any(array($key)));
-        $this->assertTrue($cache->has_all(array($key)));
+            // Recheck the read methods.
+            $this->assertEquals($datascalar, $cache->get($key));
+            $this->assertTrue($cache->has($key));
+            $result = $cache->get_many(array($key));
+            $this->assertCount(1, $result);
+            $this->assertEquals($datascalar, reset($result));
+            $this->assertTrue($cache->has_any(array($key)));
+            $this->assertTrue($cache->has_all(array($key)));
 
-        // Delete it.
-        $this->assertTrue($cache->delete($key));
+            // Delete it.
+            $this->assertTrue($cache->delete($key));
 
-        // Check its gone.
-        $this->assertFalse($cache->get($key));
-        $this->assertFalse($cache->has($key));
+            // Check its gone.
+            $this->assertFalse($cache->get($key));
+            $this->assertFalse($cache->has($key));
+        }
 
         // Test arrays.
         $this->assertTrue($cache->set($key, $dataarray));
@@ -261,11 +263,13 @@ class cache_phpunit_tests extends advanced_testcase {
         }
 
         // Test set many.
-        $cache->set_many(array('key1' => 'data1', 'key2' => 'data2'));
+        $cache->set_many(array('key1' => 'data1', 'key2' => 'data2', 'key3' => null));
         $this->assertEquals('data1', $cache->get('key1'));
         $this->assertEquals('data2', $cache->get('key2'));
+        $this->assertEquals(null, $cache->get('key3'));
         $this->assertTrue($cache->delete('key1'));
         $this->assertTrue($cache->delete('key2'));
+        $this->assertTrue($cache->delete('key3'));
 
         $cache->set_many(array(
             'key1' => array(1, 2, 3),
@@ -282,14 +286,17 @@ class cache_phpunit_tests extends advanced_testcase {
         // Test delete many.
         $this->assertTrue($cache->set('key1', 'data1'));
         $this->assertTrue($cache->set('key2', 'data2'));
+        $this->assertTrue($cache->set('key3', null));
 
         $this->assertEquals('data1', $cache->get('key1'));
         $this->assertEquals('data2', $cache->get('key2'));
+        $this->assertEquals(null, $cache->get('key3'));
 
-        $this->assertEquals(2, $cache->delete_many(array('key1', 'key2')));
+        $this->assertEquals(3, $cache->delete_many(array('key1', 'key2', 'key3')));
 
         $this->assertFalse($cache->get('key1'));
         $this->assertFalse($cache->get('key2'));
+        $this->assertFalse($cache->get('key3'));
 
         // Quick reference test.
         $obj = new stdClass;