diff --git a/site/posts/2021-05-09-variable-declarations.md b/site/posts/2021-05-09-variable-declarations.md
index a2865db..c90fcd4 100644
--- a/site/posts/2021-05-09-variable-declarations.md
+++ b/site/posts/2021-05-09-variable-declarations.md
@@ -72,7 +72,11 @@ impl Context {
 	}
 
 	fn declare_variable(&mut self, name: &str, value: Value) {
-		self.variables.insert(name.into(), value);
+		if self.variables.contains_key(name) {
+			panic!("cannot re-declare variable {}", name);
+		} else {
+			self.variables.insert(name.into(), value);
+		}
 	}
 }
 ```