diff --git a/src/evaluate/evaluate.rs b/src/evaluate/evaluate.rs
index 54b2456..2484c5a 100644
--- a/src/evaluate/evaluate.rs
+++ b/src/evaluate/evaluate.rs
@@ -43,7 +43,11 @@ pub fn evaluate(t: &Expression, context: &mut Context) -> Result<Expression, (Li
 				Expression::Quantity(_, _) => None,
 
 				Expression::Constant(_, c) => { Some(evaluate(&c.value(), context).unwrap()) },
-				Expression::Variable(_, s) => { context.get_variable(&s) },
+				Expression::Variable(l, s) => {
+					let v = context.get_variable(&s);
+					if v.is_none() { return Err((*l, EvalError::Undefined(s.clone()))); }
+					v
+				},
 				Expression::Operator(_, Operator::Function(_), _) => { Some(eval_function(g)?) },
 				Expression::Operator(_, _, _) => { eval_operator(g, context)? },
 			};
diff --git a/src/evaluate/mod.rs b/src/evaluate/mod.rs
index 59c305e..657d9b2 100644
--- a/src/evaluate/mod.rs
+++ b/src/evaluate/mod.rs
@@ -10,7 +10,8 @@ pub enum EvalError {
 	TooBig,
 	ZeroDivision,
 	IncompatibleUnit,
-	BadDefineName
+	BadDefineName,
+	Undefined(String)
 }
 
 
@@ -31,6 +32,9 @@ impl ToString for EvalError {
 			},
 			EvalError::BadDefineName => {
 				String::from("Invalid variable name")
+			},
+			EvalError::Undefined(s) => {
+				format!("{} is undefined", s)
 			}
 		}
 	}