diff --git parse.y parse.y
index 437f534..10e5fc1 100644
--- parse.y
+++ parse.y
@@ -1917,6 +1917,16 @@ arg		: lhs '=' arg
 			$$ = dispatch2(assign, $1, dispatch2(rescue_mod, $3, $5));
 		    %*/
 		    }
+		| tCONSTANT tOP_ASGN arg
+		    {
+		    /*%%%*/
+			yyerror("constant re-assignment");
+			$$ = NEW_BEGIN(0);
+		    /*%
+			$$ = dispatch3(opassign, $1, $2, $3);
+			$$ = dispatch1(assign_error, $$);
+		    %*/
+		    }
 		| var_lhs tOP_ASGN arg
 		    {
 		    /*%%%*/