Check for null argument in returnStatement parsing

instrumentSolidity.js

@@ -204,8 +204,10 @@ module.exports = function(contract, fileName, instrumentingActive){
 
 	parse["ReturnStatement"] = function(expression, instrument){
 		if (instrument){instrumentStatement(expression)}
+		if (expression.argument){
 			parse[expression.argument.type](expression.argument, instrument);
 		}
+	}
 
 	parse["NewExpression"] = function(expression, instrument){
 		parse[expression.callee.type](expression.callee, instrument);