diff --git a/tgt-vhdl/expr.cc b/tgt-vhdl/expr.cc
index c57f951a1..8de363b86 100644
--- a/tgt-vhdl/expr.cc
+++ b/tgt-vhdl/expr.cc
@@ -158,6 +158,7 @@ static vhdl_expr *translate_unary(ivl_expr_t e)
       return new vhdl_unaryop_expr
          (VHDL_UNARYOP_NOT, operand, new vhdl_type(*operand->get_type()));
    case '-':
+      operand = change_signedness(operand, true);
       return new vhdl_unaryop_expr
          (VHDL_UNARYOP_NEG, operand, new vhdl_type(*operand->get_type()));
    case 'N':   // NOR