diff --git a/lib/classes/plugininfo/filter.php b/lib/classes/plugininfo/filter.php
index 6eb4338f864..cb50ce80bb5 100644
--- a/lib/classes/plugininfo/filter.php
+++ b/lib/classes/plugininfo/filter.php
@@ -112,11 +112,25 @@ class filter extends base {
      * @private
      */
     public function uninstall_cleanup() {
-        global $DB;
+        global $DB, $CFG;
 
         $DB->delete_records('filter_active', array('filter' => $this->name));
         $DB->delete_records('filter_config', array('filter' => $this->name));
 
+        if (empty($CFG->filterall)) {
+            $stringfilters = array();
+        } else if (!empty($CFG->stringfilters)) {
+            $stringfilters = explode(',', $CFG->stringfilters);
+            $stringfilters = array_combine($stringfilters, $stringfilters);
+        } else {
+            $stringfilters = array();
+        }
+
+        unset($stringfilters[$this->name]);
+
+        set_config('stringfilters', implode(',', $stringfilters));
+        set_config('filterall', !empty($stringfilters));
+
         parent::uninstall_cleanup();
     }
 }
diff --git a/lib/filterlib.php b/lib/filterlib.php
index a93a74b87b1..5f848ec604f 100644
--- a/lib/filterlib.php
+++ b/lib/filterlib.php
@@ -721,13 +721,23 @@ function filter_get_string_filters() {
  */
 function filter_set_applies_to_strings($filter, $applytostrings) {
     $stringfilters = filter_get_string_filters();
-    $numstringfilters = count($stringfilters);
+    $prevfilters = $stringfilters;
+    $allfilters = core_component::get_plugin_list('filter');
+
     if ($applytostrings) {
         $stringfilters[$filter] = $filter;
     } else {
         unset($stringfilters[$filter]);
     }
-    if (count($stringfilters) != $numstringfilters) {
+
+    // Remove missing filters.
+    foreach ($stringfilters as $filter) {
+        if (!isset($allfilters[$filter])) {
+            unset($stringfilters[$filter]);
+        }
+    }
+
+    if ($prevfilters != $stringfilters) {
         set_config('stringfilters', implode(',', $stringfilters));
         set_config('filterall', !empty($stringfilters));
     }
diff --git a/lib/tests/filterlib_test.php b/lib/tests/filterlib_test.php
index 8dfdd0784e9..2db0568e2e0 100644
--- a/lib/tests/filterlib_test.php
+++ b/lib/tests/filterlib_test.php
@@ -606,23 +606,39 @@ class core_filterlib_testcase extends advanced_testcase {
 
     public function test_set() {
         global $CFG;
+
+        $this->assertFileExists("$CFG->dirroot/filter/emailprotect"); // Any standard filter.
+        $this->assertFileExists("$CFG->dirroot/filter/tidy");         // Any standard filter.
+        $this->assertFileNotExists("$CFG->dirroot/filter/grgrggr");   // Any non-existent filter
+
         // Setup fixture.
-        $CFG->filterall = 0;
-        $CFG->stringfilters = '';
+        set_config('filterall', 0);
+        set_config('stringfilters', '');
         // Exercise SUT.
-        filter_set_applies_to_strings('name', true);
+        filter_set_applies_to_strings('tidy', true);
         // Validate.
-        $this->assertEquals('name', $CFG->stringfilters);
+        $this->assertEquals('tidy', $CFG->stringfilters);
+        $this->assertEquals(1, $CFG->filterall);
+
+        filter_set_applies_to_strings('grgrggr', true);
+        $this->assertEquals('tidy', $CFG->stringfilters);
+        $this->assertEquals(1, $CFG->filterall);
+
+        filter_set_applies_to_strings('emailprotect', true);
+        $this->assertEquals('tidy,emailprotect', $CFG->stringfilters);
         $this->assertEquals(1, $CFG->filterall);
     }
 
     public function test_unset_to_empty() {
         global $CFG;
+
+        $this->assertFileExists("$CFG->dirroot/filter/tidy"); // Any standard filter.
+
         // Setup fixture.
-        $CFG->filterall = 1;
-        $CFG->stringfilters = 'name';
+        set_config('filterall', 1);
+        set_config('stringfilters', 'tidy');
         // Exercise SUT.
-        filter_set_applies_to_strings('name', false);
+        filter_set_applies_to_strings('tidy', false);
         // Validate.
         $this->assertEquals('', $CFG->stringfilters);
         $this->assertEquals('', $CFG->filterall);
@@ -630,13 +646,18 @@ class core_filterlib_testcase extends advanced_testcase {
 
     public function test_unset_multi() {
         global $CFG;
+
+        $this->assertFileExists("$CFG->dirroot/filter/emailprotect"); // Any standard filter.
+        $this->assertFileExists("$CFG->dirroot/filter/tidy");         // Any standard filter.
+        $this->assertFileExists("$CFG->dirroot/filter/multilang");    // Any standard filter.
+
         // Setup fixture.
-        $CFG->filterall = 1;
-        $CFG->stringfilters = 'name,other';
+        set_config('filterall', 1);
+        set_config('stringfilters', 'emailprotect,tidy,multilang');
         // Exercise SUT.
-        filter_set_applies_to_strings('name', false);
+        filter_set_applies_to_strings('tidy', false);
         // Validate.
-        $this->assertEquals('other', $CFG->stringfilters);
+        $this->assertEquals('emailprotect,multilang', $CFG->stringfilters);
         $this->assertEquals(1, $CFG->filterall);
     }
 }