diff --git a/src/crypto/tls/key_agreement.go b/src/crypto/tls/key_agreement.go
index 7e6534b..22f1b2e 100644
--- a/src/crypto/tls/key_agreement.go
+++ b/src/crypto/tls/key_agreement.go
@@ -67,7 +67,11 @@
 		return nil, nil, err
 	}
 
-	encrypted, err := rsa.EncryptPKCS1v15(config.rand(), cert.PublicKey.(*rsa.PublicKey), preMasterSecret)
+	rsaKey, ok := cert.PublicKey.(*rsa.PublicKey)
+	if !ok {
+		return nil, nil, errors.New("tls: server certificate contains incorrect key type for selected ciphersuite")
+	}
+	encrypted, err := rsa.EncryptPKCS1v15(config.rand(), rsaKey, preMasterSecret)
 	if err != nil {
 		return nil, nil, err
 	}