diff --git a/File/MARC.php b/File/MARC.php
index 19a85a8..f2ca3a3 100644
--- a/File/MARC.php
+++ b/File/MARC.php
@@ -167,9 +167,13 @@ function __construct($source, $type = self::SOURCE_FILE, $record_class = null)
             }
             break;
 
-        case self::SOURCE_STRING:
+        case self::SOURCE_STRING :
             $this->type = self::SOURCE_STRING;
+            if ($source===null)
+            $this->source=[]; 
+            else 
             $this->source = explode(File_MARC::END_OF_RECORD, $source);
+        
             break;
 
         default:
@@ -196,7 +200,9 @@ function nextRaw()
             $record = preg_replace('/^[\\x0a\\x0d\\x00]+/', "", $record);
 
         } elseif ($this->type == self::SOURCE_STRING) {
+            
             $record = array_shift($this->source);
+            
         }
 
         // Exit if we are at the end of the file
@@ -318,9 +324,9 @@ private function _decode($text)
             if (!is_numeric($offset)) {
                 $offset = 0;
             }
-
+            if ($tag===null) $tag=''; 
             // Check directory validity
-            if (!preg_match("/^[0-9A-Za-z]{3}$/", $tag)) {
+            if (  !preg_match("/^[0-9A-Za-z]{3}$/", $tag)) {
                 $marc->addWarning(File_MARC_Exception::formatError(File_MARC_Exception::$messages[File_MARC_Exception::ERROR_INVALID_DIRECTORY_TAG], array("tag" => $tag)));
             }
             if (!preg_match("/^\d{4}$/", $len)) {
diff --git a/File/MARC/List.php b/File/MARC/List.php
index 1bc9930..b9c0146 100644
--- a/File/MARC/List.php
+++ b/File/MARC/List.php
@@ -98,7 +98,9 @@ function key(): int
         if ($this->current() instanceof File_MARC_Field) {
             return $this->current()->getTag();
         } elseif ($this->current() instanceof File_MARC_Subfield) {
-            return $this->current()->getCode();
+            if (!is_int( $this->current()->getCode())) return  ord($this->current()->getcode());
+            else
+            return  $this->current()->getCode();
         }
         return false;
     }
diff --git a/File/MARC/Subfield.php b/File/MARC/Subfield.php
index 227d4af..9f43f49 100644
--- a/File/MARC/Subfield.php
+++ b/File/MARC/Subfield.php
@@ -237,7 +237,8 @@ function setPosition($pos)
     function isEmpty()
     {
         // There is data
-        if (strlen($this->data)) {
+        if ($this->data===null) return true; 
+        if ( strlen($this->data)) {
             return false;
         }
 
diff --git a/tests/marc_007.phpt b/tests/marc_007.phpt
index 5c2172b..d8eb62f 100644
--- a/tests/marc_007.phpt
+++ b/tests/marc_007.phpt
@@ -10,13 +10,14 @@ $marc_file = new File_MARC($dir . '/' . 'example.mrc');
 
 if ($marc_record = $marc_file->next()) {
     foreach ($marc_record->getFields() as $tag=>$value) {
-        print "$tag: ";
+        //print "$tag: ";
+         print $value->gettag().': '; 
 	    if ($value instanceof File_MARC_Control_Field) {
                 print $value->getData();
             }
 	    else {
                 foreach ($value->getSubfields() as $code=>$subdata) {
-                    print "_$code";
+                      print "_".$subdata->getcode();
                 }
             }
         print "\n";
diff --git a/tests/marc_list_001.phpt b/tests/marc_list_001.phpt
index 84361e6..c1ce419 100644
--- a/tests/marc_list_001.phpt
+++ b/tests/marc_list_001.phpt
@@ -50,12 +50,13 @@ echo $list->count() . " nodes\n";
 // Check key method
 echo "---\n";
 $list->rewind();
-echo 'Key (field): ' . $list->key() . "\n";
+echo 'Key (field): ' . str_pad((string)$list->key(),3,'0',STR_PAD_LEFT) . "\n";
 
 $subfield = new File_MARC_Subfield( 'a', 'test' );
 $list->prependNode( $subfield );
 $list->rewind();
-echo 'Key (subfield): ' . $list->key(). "\n";
+// when we call key() method directly we must to use chr to get the int index as char
+echo 'Key (subfield): ' . chr($list->key()). "\n";
 ?>
 --EXPECT--
 0: 002     00002