diff --git a/linkcheck/checker/fileurl.py b/linkcheck/checker/fileurl.py
index 6e0753e2..8f0134e3 100644
--- a/linkcheck/checker/fileurl.py
+++ b/linkcheck/checker/fileurl.py
@@ -234,8 +234,7 @@ class FileUrl(urlbase.UrlBase):
         with links to the files."""
         if self.is_directory():
             data = get_index_html(get_files(self.get_os_filename()))
-            if isinstance(data, str):
-                data = data.encode("iso8859-1", "ignore")
+            data = data.encode("iso8859-1", "ignore")
         else:
             data = super().read_content()
         return data
diff --git a/linkcheck/fileutil.py b/linkcheck/fileutil.py
index 5c8c8948..cc39342d 100644
--- a/linkcheck/fileutil.py
+++ b/linkcheck/fileutil.py
@@ -70,7 +70,7 @@ else:
 
 def path_safe(path):
     """Ensure path string is compatible with the platform file system encoding."""
-    if isinstance(path, str) and not os.path.supports_unicode_filenames:
+    if path and not os.path.supports_unicode_filenames:
         path = path.encode(FSCODING, "replace").decode(FSCODING)
     return path
 
diff --git a/linkcheck/robotparser2.py b/linkcheck/robotparser2.py
index d4ca6ee2..1a5af387 100644
--- a/linkcheck/robotparser2.py
+++ b/linkcheck/robotparser2.py
@@ -276,10 +276,6 @@ class RobotFileParser:
             useragent,
             url,
         )
-        if not isinstance(useragent, str):
-            useragent = useragent.encode("ascii", "ignore")
-        if not isinstance(url, str):
-            url = url.encode("ascii", "ignore")
         if self.disallow_all:
             log.debug(LOG_CHECK, " ... disallow all.")
             return False
diff --git a/linkcheck/strformat.py b/linkcheck/strformat.py
index 62338042..aced2d42 100644
--- a/linkcheck/strformat.py
+++ b/linkcheck/strformat.py
@@ -39,12 +39,12 @@ def ascii_safe(s):
     """Get ASCII string without raising encoding errors. Unknown
     characters of the given encoding will be ignored.
 
-    @param s: the Unicode string to be encoded
-    @type s: unicode or None
-    @return: encoded ASCII version of s, or None if s was None
-    @rtype: string
+    @param s: the string to be encoded
+    @type s: string or None
+    @return: version of s containing only ASCII characters, or None if s was None
+    @rtype: string or None
     """
-    if isinstance(s, str):
+    if s:
         s = s.encode('ascii', 'ignore').decode('ascii')
     return s
 
diff --git a/linkcheck/url.py b/linkcheck/url.py
index 13ae3f91..eb8fc171 100644
--- a/linkcheck/url.py
+++ b/linkcheck/url.py
@@ -183,7 +183,7 @@ def idna_encode(host):
     to RFC 3490.
     @raise: UnicodeError if hostname is not properly IDN encoded.
     """
-    if host and isinstance(host, str):
+    if host:
         try:
             host.encode('ascii')
             return host, False
@@ -255,7 +255,7 @@ def url_fix_common_typos(url):
 
 def url_fix_mailto_urlsplit(urlparts):
     """Split query part of mailto url if found."""
-    sep = b"?" if isinstance(urlparts[2], bytes) else "?"
+    sep = "?"
     if sep in urlparts[2]:
         urlparts[2], urlparts[3] = urlparts[2].split(sep, 1)