diff --git a/README.md b/README.md
index bf7d2ce..a9a91b1 100644
--- a/README.md
+++ b/README.md
@@ -19,13 +19,13 @@ git = "https://github.com/TimDumol/rust-otp"
 
 ```rust
 // first argument is the secret, second argument is the counter
-assert_eq!(make_hotp("base32secret3232".to_ascii(), 0), Some(260182));
+assert_eq!(make_hotp("base32secret3232".to_ascii_uppercase().ok(), 0), Some(260182));
 
 // first argument is the secret, followed by the time step in seconds (Google
 // Authenticator uses a time step of 30), and then the skew in seconds
 // (often used when calculating HOTPs for a sequence of consecutive
 // time intervals, to deal with potential latency and desynchronization).
-assert_eq!(make_totp("base32secret3232".to_ascii(), 30, 0), Some(260182)); // true on Unix epoch
+assert_eq!(make_totp("base32secret3232".to_ascii_uppercase(), 30, 0).ok(), Some(260182)); // true on Unix epoch
 ```
 
 
diff --git a/src/lib.rs b/src/lib.rs
index 0d7445c..28d39be 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -71,7 +71,7 @@ pub fn make_totp(secret: &str, time_step: u64, skew: i64) -> Result<u32, Error>
 
 #[cfg(test)]
 mod tests {
-    use super::{make_hotp, make_totp_helper};
+    use super::{make_hotp, make_totp, make_totp_helper, Error};
 
     #[test]
     fn hotp() {
@@ -86,5 +86,11 @@ mod tests {
         assert_eq!(make_totp_helper("BASE32SECRET3232", 3600, 0, 7).unwrap(), 260182);
         assert_eq!(make_totp_helper("BASE32SECRET3232", 30, 0, 35).unwrap(), 55283);
         assert_eq!(make_totp_helper("BASE32SECRET3232", 1, -2, 1403).unwrap(), 316439);
+        assert!(
+            matches!(
+                make_totp("base32secret3232", 30, 0).unwrap_err(),
+                Error::InvalidSecret{..}
+            )
+        );
     }
 }