diff --git a/src/monomorph.rs b/src/monomorph.rs
index 4f7ecc8..c046dcb 100644
--- a/src/monomorph.rs
+++ b/src/monomorph.rs
@@ -557,7 +557,14 @@ fn mono_expr(
             ast::Expr::Char(*char)
         }
 
-        ast::Expr::Constr(_) | ast::Expr::Self_ => expr.clone(),
+        ast::Expr::Constr(constr) => {
+            let ty_decl = poly_pgm.ty.get(constr).unwrap();
+            let mono_constr = mono_ty_decl(ty_decl, &[], poly_pgm, mono_pgm);
+            debug_assert_eq!(&mono_constr, constr);
+            ast::Expr::Constr(mono_constr)
+        }
+
+        ast::Expr::Self_ => ast::Expr::Self_,
 
         ast::Expr::Call(ast::CallExpr { fun, args }) => ast::Expr::Call(ast::CallExpr {
             fun: mono_bl_expr(fun, ty_map, poly_pgm, mono_pgm),