diff --git a/src/OpenSSL/crypto.py b/src/OpenSSL/crypto.py
index 8e83749..e72e857 100644
--- a/src/OpenSSL/crypto.py
+++ b/src/OpenSSL/crypto.py
@@ -1642,8 +1642,7 @@
         raise ValueError("type argument must be FILETYPE_PEM or FILETYPE_ASN1")
 
     result_code = write_bio(bio, pkey._pkey)
-    if result_code != 1:
-        # TODO: This is untested.
+    if result_code != 1:  # pragma: no cover
         _raise_current_error()
 
     return _bio_to_string(bio)