diff --git a/key.go b/key.go
index 37afc24..d2230fd 100644
--- a/key.go
+++ b/key.go
@@ -572,10 +572,6 @@ func (k *Key) PrivateKey() (crypto.PrivateKey, error) {
 		return nil, err
 	}
 
-	if len(k.D) == 0 {
-		return nil, ErrNotPrivKey
-	}
-
 	switch alg {
 	case AlgorithmES256, AlgorithmES384, AlgorithmES512:
 		// RFC8152 allows omitting X and Y from private keys;
diff --git a/key_test.go b/key_test.go
index c2d2979..a65598b 100644
--- a/key_test.go
+++ b/key_test.go
@@ -612,6 +612,12 @@ func TestKey_PrivateKey(t *testing.T) {
 	_, err = k.PrivateKey()
 	assertEqualError(t, err, ErrNotPrivKey.Error())
 
+	k.KeyType = KeyTypeEC2
+	k.Curve = CurveP256
+
+	_, err = k.PrivateKey()
+	assertEqualError(t, err, ErrNotPrivKey.Error())
+
 	k = Key{
 		KeyType: KeyTypeOKP,
 		Curve:   CurveEd25519,
@@ -621,11 +627,19 @@ func TestKey_PrivateKey(t *testing.T) {
 	_, err = k.PrivateKey()
 	assertEqualError(t, err, ErrOKPNoPub.Error())
 
+	k.Curve = CurveInvalid
+	_, err = k.PrivateKey()
+	assertEqualError(t, err, ErrInvalidKey.Error())
+
 	k.KeyType = KeyTypeEC2
 	k.Curve = CurveP256
 
 	_, err = k.PrivateKey()
 	assertEqualError(t, err, ErrEC2NoPub.Error())
+
+	k.Curve = CurveInvalid
+	_, err = k.PrivateKey()
+	assertEqualError(t, err, ErrInvalidKey.Error())
 }
 
 func TestKey_PublicKey(t *testing.T) {