diff --git a/core/src/main/java/org/bitcoinj/crypto/HDKeyDerivation.java b/core/src/main/java/org/bitcoinj/crypto/HDKeyDerivation.java
index 80d8c807..55d96a3e 100644
--- a/core/src/main/java/org/bitcoinj/crypto/HDKeyDerivation.java
+++ b/core/src/main/java/org/bitcoinj/crypto/HDKeyDerivation.java
@@ -186,7 +186,7 @@ public final class HDKeyDerivation {
     }
 
     public static RawKeyBytes deriveChildKeyBytesFromPublic(DeterministicKey parent, ChildNumber childNumber, PublicDeriveMode mode) throws HDDerivationException {
-        checkArgument(!childNumber.isHardened(), "Can't use private derivation with public keys only.");
+        checkArgument(!childNumber.isHardened(), "Hardened derivation is unsupported (%s).", childNumber);
         byte[] parentPublicKey = parent.getPubKeyPoint().getEncoded(true);
         checkState(parentPublicKey.length == 33, "Parent pubkey must be 33 bytes, but is " + parentPublicKey.length);
         ByteBuffer data = ByteBuffer.allocate(37);