Index: wp-includes/pomo/entry.php
===================================================================
--- wp-includes/pomo/entry.php	(revision 19601)
+++ wp-includes/pomo/entry.php	(working copy)
@@ -61,9 +61,9 @@
 	 * @return string|bool the key or false if the entry is empty
 	 */
 	function key() {
-		if (is_null($this->singular)) return false;
+		if ( null === $this->singular ) return false;
 		// prepend context and EOT, like in MO files
-		return is_null($this->context)? $this->singular : $this->context.chr(4).$this->singular;
+		return ( null === $this->context ) ? $this->singular : $this->context . chr(4) . $this->singular;
 	}
 	
 	function merge_with(&$other) {