diff --git a/plugins/sources/spreadsheet.inc b/plugins/sources/spreadsheet.inc
index b7b6de7..5925b36 100644
--- a/plugins/sources/spreadsheet.inc
+++ b/plugins/sources/spreadsheet.inc
@@ -208,12 +208,18 @@ class MigrateSourceSpreadsheet extends MigrateSource {
     migrate_instrument_start('MigrateSourceSpreadsheet::next');
     ++$this->rowNumber;
 
-    $source_keys = array_keys($this->activeMap->getSourceKey());
+    // Only take into account the fields actually used in source.
+    $mappings = Migration::currentMigration()->getFieldMappings();
+    $source_fields = array();
+    foreach ($mappings as $mapping) {
+      $source_fields[] = $mapping->getSourceField();
+    }
+
 
     if ($this->rowNumber <= $this->rows) {
       $row_values = array();
       for ($col = 0; $col < $this->cols; ++$col) {
-        if (in_array($this->fields[$col], $source_keys)) {
+        if (in_array($this->fields[$col], $source_fields)) {
           $row_values[$this->fields[$col]] = trim($this->worksheet->getCellByColumnAndRow($col, $this->rowNumber)->getValue());
         }
       }
@@ -228,5 +234,4 @@ class MigrateSourceSpreadsheet extends MigrateSource {
       return NULL;
     }
   }
-
 }