diff --git a/src/context.rs b/src/context.rs
index f6d3af1..f44d8be 100644
--- a/src/context.rs
+++ b/src/context.rs
@@ -13,19 +13,29 @@ impl Context {
 	}
 
 	pub fn push_hist(&mut self, t: Expression) { self.history.push(t); }
-	pub fn push_var(&mut self, s: String, t: Expression) { self.variables.insert(s, t); }
+	pub fn push_var(&mut self, s: String, t: Expression) -> Result<(), ()> {
+		if self.valid_varible(&s) {
+			self.variables.insert(s, t);
+			return Ok(());
+		} else { return Err(()); }
+	}
 	pub fn del_var(&mut self, s: &String) { self.variables.remove(s); }
 
 	pub fn get_variable(&self, s: &String) -> Option<Expression> {
-
 		let v: Option<&Expression>;
-
 		if s == "ans" {
 			v = self.history.last();
 		} else {
 			v = self.variables.get(s);
 		}
-
 		if v.is_some() { Some(v.unwrap().clone()) } else { None }
 	}
+
+	pub fn valid_varible(&self, s: &str) -> bool {
+		return match s {
+			"ans" => false,
+			_ => true
+		}
+	}
+
 }
diff --git a/src/evaluate/evaluate.rs b/src/evaluate/evaluate.rs
index cc5cdb1..b6c699c 100644
--- a/src/evaluate/evaluate.rs
+++ b/src/evaluate/evaluate.rs
@@ -59,14 +59,16 @@ pub fn evaluate(t: &Expression, context: &mut Context) -> Result<Expression, Eva
 
 		} else {
 			// Move down the tree
-			coords.push(0);
 
-			let n = root.get_at_coords(&coords[..]);
-			if let Some(n) = n {
-				if let Expression::Operator(Operator::Define, _) = n {
+			// Don't evaluate the first argument of a define.
+			// This prevents variables from being expanded before a re-assignment.
+			if let Expression::Operator(Operator::Define, _) = g {
 					*coords.last_mut().unwrap() += 1;
+				coords.push(0);
+				continue;
 				}
-			}
+
+			coords.push(0);
 		}
 	}
 
diff --git a/src/evaluate/operator.rs b/src/evaluate/operator.rs
index a21d88a..a371850 100644
--- a/src/evaluate/operator.rs
+++ b/src/evaluate/operator.rs
@@ -15,7 +15,8 @@ pub fn eval_operator(op: &Operator, args: &VecDeque<Expression>, context: &mut C
 			let b = &args[1];
 
 			if let Expression::Variable(s) = &args[0] {
-				context.push_var(s.clone(), b.clone());
+				let r = context.push_var(s.clone(), b.clone());
+				if r.is_err() { return Err(EvalError::BadDefineName); }
 				return Ok(Some(b.clone()));
 			} else { return Err(EvalError::BadDefineName); }
 		},