diff --git a/src/rpn.rs b/src/rpn.rs
index 0cb23b8..8ccf5d6 100644
--- a/src/rpn.rs
+++ b/src/rpn.rs
@@ -52,8 +52,10 @@ use std::{
 
 use CalcType::*;
 
+extern crate strerror;
 extern crate sysexits;
 
+use strerror::c_error;
 use sysexits::EX_DATAERR;
 
 #[derive(Clone, PartialEq, PartialOrd, Debug)]
@@ -172,7 +174,7 @@ fn eval(
 					};
 				} else {
 					return Err(EvaluationError {
-						message: format!("{}: Unexpected operation.", op),
+						message: format!("{}: Unexpected operation", op),
 						code: EX_DATAERR,
 					})
 				}