Index: library/Zend/Filter/StringToLower.php
===================================================================
--- library/Zend/Filter/StringToLower.php	(revision 22663)
+++ library/Zend/Filter/StringToLower.php	(working copy)
@@ -57,6 +57,12 @@
             $options = $temp;
         }
 
+        if (!array_key_exists('encoding', $options)
+            && function_exists('mb_internal_encoding'))
+        {
+            $options['encoding'] = mb_internal_encoding();
+        }
+
         if (array_key_exists('encoding', $options)) {
             $this->setEncoding($options['encoding']);
         }
Index: library/Zend/Filter/StringToUpper.php
===================================================================
--- library/Zend/Filter/StringToUpper.php	(revision 22663)
+++ library/Zend/Filter/StringToUpper.php	(working copy)
@@ -57,6 +57,12 @@
             $options = $temp;
         }
 
+        if (!array_key_exists('encoding', $options)
+            && function_exists('mb_internal_encoding'))
+        {
+            $options['encoding'] = mb_internal_encoding();
+        }
+
         if (array_key_exists('encoding', $options)) {
             $this->setEncoding($options['encoding']);
         }
Index: tests/Zend/Filter/StringToLowerTest.php
===================================================================
--- tests/Zend/Filter/StringToLowerTest.php	(revision 22663)
+++ tests/Zend/Filter/StringToLowerTest.php	(working copy)
@@ -169,4 +169,15 @@
             $this->assertContains('mbstring is required', $e->getMessage());
         }
     }
+
+    /**
+     * @group ZF-9854
+     */
+    public function testDetectMbInternalEncoding()
+    {
+        if (!function_exists('mb_internal_encoding')) {
+            $this->markTestSkipped("Not detected the function 'mb_internal_encoding'");
+        }
+        $this->assertEquals(mb_internal_encoding(), $this->_filter->getEncoding());
+    }
 }
Index: tests/Zend/Filter/StringToUpperTest.php
===================================================================
--- tests/Zend/Filter/StringToUpperTest.php	(revision 22663)
+++ tests/Zend/Filter/StringToUpperTest.php	(working copy)
@@ -169,4 +169,15 @@
             $this->assertContains('mbstring is required', $e->getMessage());
         }
     }
+
+    /**
+     * @group ZF-9854
+     */
+    public function testDetectMbInternalEncoding()
+    {
+        if (!function_exists('mb_internal_encoding')) {
+            $this->markTestSkipped("Not detected the function 'mb_internal_encoding'");
+        }
+        $this->assertEquals(mb_internal_encoding(), $this->_filter->getEncoding());
+    }
 }