Index: class.ux_t3lib_db.php
===================================================================
--- class.ux_t3lib_db.php	(revision 29771)
+++ class.ux_t3lib_db.php	(working copy)
@@ -2729,7 +2729,7 @@
 					$t = $sqlPartArray[$k]['table'] ? $sqlPartArray[$k]['table'] : $defaultTable;
 
 						// Mapping field name, if set:
-					if (is_array($this->mapping[$t]['mapFieldNames']) && $this->mapping[$t]['mapFieldNames'][$sqlPartArray[$k]['field']]) {
+					if (is_array($this->mapping[$t]['mapFieldNames']) && isset($this->mapping[$t]['mapFieldNames'][$sqlPartArray[$k]['field']])) {
 						$sqlPartArray[$k]['field'] = $this->mapping[$t]['mapFieldNames'][$sqlPartArray[$k]['field']];
 					}
 
Index: tests/db_mssql_testcase.php
===================================================================
--- tests/db_mssql_testcase.php	(revision 29770)
+++ tests/db_mssql_testcase.php	(working copy)
@@ -126,6 +126,29 @@
 	}
 
 	///////////////////////////////////////
+	// Tests concerning remapping with
+	// external (non-TYPO3) databases
+	///////////////////////////////////////
+
+	/**
+	 * @test
+	 * @see http://bugs.typo3.org/view.php?id=13490
+	 */
+	public function canRemapPidToZero() {
+		$selectFields = 'uid, FirstName, LastName';
+		$fromTables   = 'Members';
+		$whereClause  = 'pid=0 AND cruser_id=1';
+		$groupBy      = '';
+		$orderBy      = '';
+
+		$GLOBALS['TYPO3_DB']->_callRef('map_remapSELECTQueryParts', $selectFields, $fromTables, $whereClause, $groupBy, $orderBy);
+		$query = $this->cleanSql($GLOBALS['TYPO3_DB']->SELECTquery($selectFields, $fromTables, $whereClause, $groupBy, $orderBy));
+
+		$expected = 'SELECT "MemberID", "FirstName", "LastName" FROM "Members" WHERE 0 = 0 AND 1 = 1';
+		$this->assertEquals($expected, $query);
+	}
+
+	///////////////////////////////////////
 	// Tests concerning advanced operators
 	///////////////////////////////////////
 
Index: tests/fixtures/mssql.config.php
===================================================================
--- tests/fixtures/mssql.config.php	(revision 29770)
+++ tests/fixtures/mssql.config.php	(working copy)
@@ -27,5 +27,12 @@
 			'datastructure' => 'ds',
 		),
 	),
+	'Members' => array(
+		'mapFieldNames' => array(
+			'pid' => '0',
+			'cruser_id' => '1',
+			'uid' => 'MemberID',
+		),
+	),
 );
 ?>
\ No newline at end of file