diff --git a/runtime/include/wii_es_crypto.h b/runtime/include/wii_es_crypto.h
index d1fb78c..0c515b5 100644
--- a/runtime/include/wii_es_crypto.h
+++ b/runtime/include/wii_es_crypto.h
@@ -132,8 +132,9 @@ using CryptoEcdsa = CryptoPP::ECDSA<CryptoPP::EC2N, CryptoPP::SHA1>;
 
 inline CryptoEcdsa::PrivateKey MakePrivateKey(const uint8_t* key) {
     CryptoPP::DL_GroupParameters_EC<CryptoPP::EC2N> parameters(CryptoPP::ASN1::sect233r1());
-    const CryptoPP::Integer exponent(key, 30);
-    if (exponent <= CryptoPP::Integer::Zero() || exponent >= parameters.GetSubgroupOrder()) {
+    // Wii keys need not be canonical scalars; reduction preserves their public key.
+    const CryptoPP::Integer exponent = CryptoPP::Integer(key, 30) % parameters.GetSubgroupOrder();
+    if (exponent == CryptoPP::Integer::Zero()) {
         throw std::invalid_argument("Wii ES private key is outside the sect233r1 subgroup");
     }