diff --git a/docs/tagged_union_pattern_in_typescript.md b/docs/tagged_union_pattern_in_typescript.md
index 59238b9..0ab3a63 100644
--- a/docs/tagged_union_pattern_in_typescript.md
+++ b/docs/tagged_union_pattern_in_typescript.md
@@ -94,8 +94,10 @@ class Expr {
                 return k.value;
             case "Add":
                 return k.right.eval() + k.left.eval();
+            default:
+                k satisfies never; // compile time exhaustiveness check
+                throw new Error("unexhausted"): // run time exhaustiveness check
         }
-        k satisfies never; // compile time exhaustiveness check
     }
     // ...
 }
@@ -195,13 +197,14 @@ class Expr {
         const k = this.kind;
         switch (k.tag) {
             case "Int":
-                return;
+                break;
             case "Add":
                 k.left.visit(v);
                 k.right.visit(v);
-                return
+                break;
+            default:
+                k satisfies never;
         }
-        k satisfies never;
     }
     // ...
 }
diff --git a/src/mir_interpreter.ts b/src/mir_interpreter.ts
index 9b5e4ba..54c98ec 100644
--- a/src/mir_interpreter.ts
+++ b/src/mir_interpreter.ts
@@ -157,8 +157,9 @@ class Val {
                 return `${k.value}`;
             case "Fn":
                 return `<${k.fn.ty.pretty()}>`;
+            default:
+                k satisfies never;
         }
-        k satisfies never;
     }
 }