diff --git a/Ed25519/Signer.cs b/Ed25519/Signer.cs
index aa85ea0..82adeff 100644
--- a/Ed25519/Signer.cs
+++ b/Ed25519/Signer.cs
@@ -10,8 +10,8 @@ namespace Ed25519
     {
         public static ReadOnlySpan<byte> Sign(ReadOnlySpan<byte> message, ReadOnlySpan<byte> privateKey, ReadOnlySpan<byte> publicKey)
         {
-            if(privateKey.Length == 0)
-                throw new ArgumentException("Private key length is wrong. Key must not be empty.");
+            if(privateKey.Length != Constants.BIT_LENGTH / 8)
+                throw new ArgumentException($"Private key length is wrong. Got {publicKey.Length} instead of {Constants.BIT_LENGTH / 8}.");
 
             if (publicKey.Length != Constants.BIT_LENGTH / 8)
                 throw new ArgumentException($"Public key length is wrong. Got {publicKey.Length} instead of {Constants.BIT_LENGTH / 8}.");