diff --git a/src/Mustache/Engine.php b/src/Mustache/Engine.php
index 38c05c9..41cb920 100644
--- a/src/Mustache/Engine.php
+++ b/src/Mustache/Engine.php
@@ -611,12 +611,6 @@ class Mustache_Engine
      */
     public function getTemplateClassName($source)
     {
-        if ($source instanceof Mustache_Source) {
-            $key = $source->getKey();
-        } else {
-            $key = sprintf('source:%s', $source);
-        }
-
         // For the most part, adding a new option here should do the trick.
         //
         // Pick a value here which is unique for each possible way the template
@@ -625,17 +619,26 @@ class Mustache_Engine
         // 'default' escapes.
         //
         // Keep this list in alphabetical order :)
-        $options = array(
+        $chunks = array(
             'charset'         => $this->charset,
             'delimiters'      => $this->delimiters,
             'entityFlags'     => $this->entityFlags,
             'escape'          => isset($this->escape) ? 'custom' : 'default',
+            'key'             => ($source instanceof Mustache_Source) ? $source->getKey() : 'source',
             'pragmas'         => $this->getPragmas(),
             'strictCallables' => $this->strictCallables,
             'version'         => self::VERSION,
         );
 
-        return $this->templateClassPrefix . md5(json_encode($options) . "\n" . $key);
+        $key = json_encode($chunks);
+
+        // Template Source instances have already provided their own source key. For strings, just include the whole
+        // source string in the md5 hash.
+        if (!$source instanceof Mustache_Source) {
+            $key .= "\n" . $source;
+        }
+
+        return $this->templateClassPrefix . md5($key);
     }
 
     /**
diff --git a/src/Mustache/Source.php b/src/Mustache/Source.php
index 3eded91..e6dfdf4 100644
--- a/src/Mustache/Source.php
+++ b/src/Mustache/Source.php
@@ -17,6 +17,12 @@ interface Mustache_Source
     /**
      * Get the Source key (used to generate the compiled class name).
      *
+     * This must return a distinct key for each template source. For example, an
+     * MD5 hash of the template contents would probably do the trick. The
+     * ProductionFilesystemLoader uses mtime and file path. If your production
+     * source directory is under version control, you could use the current Git
+     * rev and the file path...
+     *
      * @throws RuntimeException when a source file cannot be read
      *
      * @return string
@@ -26,6 +32,8 @@ interface Mustache_Source
     /**
      * Get the template Source.
      *
+     * @throws RuntimeException when a source file cannot be read
+     *
      * @return string
      */
     public function getSource();
diff --git a/src/Mustache/Source/FilesystemSource.php b/src/Mustache/Source/FilesystemSource.php
index 7928ab2..1201af8 100644
--- a/src/Mustache/Source/FilesystemSource.php
+++ b/src/Mustache/Source/FilesystemSource.php
@@ -19,19 +19,19 @@
  */
 class Mustache_Source_FilesystemSource implements Mustache_Source
 {
-    private $filename;
+    private $fileName;
     private $statProps;
     private $stat;
 
     /**
      * Filesystem Source constructor.
      *
-     * @param string $filename
+     * @param string $fileName
      * @param array  $statProps
      */
-    public function __construct($filename, array $statProps)
+    public function __construct($fileName, array $statProps)
     {
-        $this->filename = $filename;
+        $this->fileName = $fileName;
         $this->statProps = $statProps;
     }
 
@@ -45,24 +45,24 @@ class Mustache_Source_FilesystemSource implements Mustache_Source
     public function getKey()
     {
         $chunks = array(
-            sprintf('filename:%s', $this->filename),
+            'fileName' => $this->fileName,
         );
 
         if (!empty($this->statProps)) {
             if (!isset($this->stat)) {
-                $this->stat = stat($this->filename);
+                $this->stat = stat($this->fileName);
             }
 
             if ($this->stat === false) {
-                throw new RuntimeException(sprintf('Failed to read source file "%s".', $this->filename));
+                throw new RuntimeException(sprintf('Failed to read source file "%s".', $this->fileName));
             }
 
             foreach ($this->statProps as $prop) {
-                $chunks[] = sprintf('%s:%s', $prop, $this->stat[$prop]);
+                $chunks[$prop] = $this->stat[$prop];
             }
         }
 
-        return implode(',', $chunks);
+        return json_encode($chunks);
     }
 
     /**
@@ -72,6 +72,6 @@ class Mustache_Source_FilesystemSource implements Mustache_Source
      */
     public function getSource()
     {
-        return file_get_contents($this->filename);
+        return file_get_contents($this->fileName);
     }
 }