diff --git a/rsa.go b/rsa.go
index a027ced..36e719a 100644
--- a/rsa.go
+++ b/rsa.go
@@ -50,9 +50,12 @@ func SupportsSignatureRSAPKCS1v15(ch crypto.Hash) (supported bool) {
 	}
 	defer C.go_openssl_EVP_PKEY_CTX_free(ctx)
 
+	msg := make([]byte, 1)
+	var outLen C.size_t
 	return C.go_openssl_EVP_PKEY_sign_init(ctx) == 1 &&
 		C.go_openssl_EVP_PKEY_CTX_ctrl(ctx, -1, -1, C.GO_EVP_PKEY_CTRL_MD, 0, unsafe.Pointer(md)) == 1 &&
-		setRSAPAdding(ctx, C.GO_RSA_PKCS1_PADDING) == nil
+		setRSAPAdding(ctx, C.GO_RSA_PKCS1_PADDING) == nil &&
+		C.go_openssl_EVP_PKEY_sign(ctx, nil, &outLen, base(msg), C.size_t(len(msg))) == 1
 }
 
 func GenerateKeyRSA(bits int) (N, E, D, P, Q, Dp, Dq, Qinv BigInt, err error) {