diff --git a/src/sshkey_tools/keys.py b/src/sshkey_tools/keys.py
index 9143cbf..46293cb 100644
--- a/src/sshkey_tools/keys.py
+++ b/src/sshkey_tools/keys.py
@@ -26,7 +26,7 @@
 from cryptography.hazmat.primitives.asymmetric import rsa as _RSA
 
 from . import exceptions as _EX
-from .utils import ensure_bytestring, ensure_string
+from .utils import ensure_bytestring, ensure_string, nullsafe_getattr
 from .utils import md5_fingerprint as _FP_MD5
 from .utils import sha256_fingerprint as _FP_SHA256
 from .utils import sha512_fingerprint as _FP_SHA512
@@ -129,12 +129,15 @@ def __init__(
             _SERIALIZATION.Encoding.OpenSSH,
             _SERIALIZATION.PublicFormat.OpenSSH,
         ]
+        
+        # Ensure comment is not None
+        self.comment = nullsafe_getattr(self, "comment", "")
 
     @classmethod
     def from_class(
         cls,
         key_class: PubkeyClasses,
-        comment: Union[str, bytes] = None,
+        comment: Union[str, bytes] = "",
         key_type: Union[str, bytes] = None,
     ) -> "PublicKey":
         """
@@ -266,7 +269,7 @@ def to_string(self, encoding: str = "utf-8") -> str:
         return " ".join(
             [
                 ensure_string(self.serialize(), encoding),
-                ensure_string(getattr(self, "comment", ""), encoding),
+                ensure_string(nullsafe_getattr(self, "comment", ""), encoding),
             ]
         )
 
diff --git a/src/sshkey_tools/utils.py b/src/sshkey_tools/utils.py
index 0d98234..4b21af2 100644
--- a/src/sshkey_tools/utils.py
+++ b/src/sshkey_tools/utils.py
@@ -233,6 +233,22 @@ def sha512_fingerprint(data: bytes, prefix: bool = True) -> str:
     )
 
 
+def nullsafe_getattr(obj, attr: str, default):
+    """
+    Null-safe getattr, ensuring the result is not None.
+    If the result is None, the default value is returned instead.
+
+    Args:
+        obj: The object
+        attr: The attribute to get
+        default: The default value
+    """
+    att = getattr(obj, attr, default)
+    if att is None:
+        att = default
+    
+    return att
+
 def join_dicts(*dicts) -> dict:
     """
     Joins two or more dictionaries together.