diff --git a/src/eval.rs b/src/eval.rs
index 80c1d24..e4b826c 100644
--- a/src/eval.rs
+++ b/src/eval.rs
@@ -85,7 +85,7 @@ impl ExprVisitor<Result<EvaluatedValue, ScriptError>> for InterpreterRunner<'_>
     ) -> Result<EvaluatedValue, ScriptError> {
         let operand = self.visit_expr(expr)?;
         match operator.kind() {
-            TokenKind::Minus => evaluate_negation(operand, operator),
+            TokenKind::Minus => evaluate_negation(&operand, operator),
             TokenKind::Bang => Ok(EvaluatedValue::Boolean(!operand.is_truthy())),
             _ => unreachable!(
                 "attempted to evaluate non-unary operator as unary operator: {:?}",
@@ -274,7 +274,7 @@ fn evaluate_addition(
         _ => Err(ScriptError {
             line: operator.line(),
             location: String::new(),
-            message: format!("left and right operands must be both numbers or both strings"),
+            message: "left and right operands must be both numbers or both strings".to_string(),
         }),
     }
 }
@@ -291,7 +291,7 @@ fn evaluate_binary_arithmetic<F: FnOnce(f64, f64) -> EvaluatedValue>(
 }
 
 fn evaluate_negation(
-    operand: EvaluatedValue,
+    operand: &EvaluatedValue,
     operator: &Token,
 ) -> Result<EvaluatedValue, ScriptError> {
     match operand {
@@ -299,7 +299,7 @@ fn evaluate_negation(
         _ => Err(ScriptError {
             line: operator.line(),
             location: String::new(),
-            message: format!("operand must be a number"),
+            message: "operand must be a number".to_string(),
         }),
     }
 }