diff --git a/docs/x509/tutorial.rst b/docs/x509/tutorial.rst
index 6e587d8..d1c8ba1 100644
--- a/docs/x509/tutorial.rst
+++ b/docs/x509/tutorial.rst
@@ -37,7 +37,7 @@
     ...     backend=default_backend()
     ... )
     >>> # Write our key to disk for safe keeping
-    >>> with open("path/to/store/key.pem", "w") as f:
+    >>> with open("path/to/store/key.pem", "wb") as f:
     ...     f.write(key.private_bytes(
     ...         encoding=serialization.Encoding.PEM,
     ...         format=serialization.PrivateFormat.TraditionalOpenSSL,
@@ -75,7 +75,7 @@
     ... # Sign the CSR with our private key.
     ... ])).sign(key, hashes.SHA256(), default_backend())
     >>> # Write our CSR out to disk.
-    >>> with open("path/to/csr.pem", "w") as f:
+    >>> with open("path/to/csr.pem", "wb") as f:
     ...     f.write(csr.public_bytes(serialization.Encoding.PEM))
 
 Now we can give our CSR to a CA, who will give a certificate to us in return.