Index: includes/parser/Preprocessor_DOM.php
===================================================================
--- includes/parser/Preprocessor_DOM.php	(revision 41298)
+++ includes/parser/Preprocessor_DOM.php	(working copy)
@@ -1320,6 +1320,9 @@
 	}
 
 	function getArgument( $index ) {
+		if ( !isset( $this->args[$index] ) ) {
+			return false;
+		}
 		return $this->args[$index];
 	}
 }
Index: includes/parser/Preprocessor_Hash.php
===================================================================
--- includes/parser/Preprocessor_Hash.php	(revision 41298)
+++ includes/parser/Preprocessor_Hash.php	(working copy)
@@ -1272,6 +1272,9 @@
 	}
 
 	function getArgument( $index ) {
+		if ( !isset( $this->args[$index] ) ) {
+			return false;
+		}
 		return $this->args[$index];
 	}
 }