diff --git a/src/cmd_hmac_sha1.rs b/src/cmd_hmac_sha1.rs
index 26164b9..2a44585 100644
--- a/src/cmd_hmac_sha1.rs
+++ b/src/cmd_hmac_sha1.rs
@@ -46,6 +46,10 @@ impl Command for CommandImpl {
             return simple_error!("Challenge must assigned");
         };
 
+        if secret_bytes.len() != 20 {
+            return simple_error!("Secret length must be 20, actual is: {}", secret_bytes.len());
+        }
+
         // Challenge can not be greater than 64 bytes
         if challenge_bytes.len() > 64 {
             return simple_error!("Challenge bytes is: {}, more than 64", challenge_bytes.len());