diff --git a/docs/changelog.md b/docs/changelog.md
index 1f4307cd..38f43e96 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * Remove legacy import needed only in Python 2 (#1403)
 * Fix typo that left the attribute `AdmonitionProcessor.content_indent` unset
   (#1404)
+* Fix edge-case crash in `InlineProcessor` with `AtomicString` (#1406).
 * Improve and expand type annotations in the code base (#1401).
 
 ## [3.5.1] -- 2023-10-31
diff --git a/markdown/treeprocessors.py b/markdown/treeprocessors.py
index dc857204..46d9c0cb 100644
--- a/markdown/treeprocessors.py
+++ b/markdown/treeprocessors.py
@@ -218,7 +218,7 @@ def linkText(text: str | None) -> None:
                         text = data[strartIndex:index]
                         linkText(text)
 
-                    if not isString(node):  # it's Element
+                    if isinstance(node, str):  # it's Element
                         for child in [node] + list(node):
                             if child.tail:
                                 if child.tail.strip():
@@ -304,7 +304,7 @@ def __applyPattern(
         if node is None:
             return data, True, end
 
-        if not isString(node):
+        if not isinstance(node, str):
             if not isinstance(node.text, util.AtomicString):
                 # We need to process current node too
                 for child in [node] + list(node):
diff --git a/tests/test_apis.py b/tests/test_apis.py
index d613a822..fbb46e60 100644
--- a/tests/test_apis.py
+++ b/tests/test_apis.py
@@ -30,6 +30,7 @@
 import markdown
 import warnings
 from markdown.__main__ import parse_options
+from markdown import inlinepatterns
 from logging import DEBUG, WARNING, CRITICAL
 import yaml
 import tempfile
@@ -664,8 +665,8 @@ class testAtomicString(unittest.TestCase):
     """ Test that `AtomicStrings` are honored (not parsed). """
 
     def setUp(self):
-        md = markdown.Markdown()
-        self.inlineprocessor = md.treeprocessors['inline']
+        self.md = markdown.Markdown()
+        self.inlineprocessor = self.md.treeprocessors['inline']
 
     def testString(self):
         """ Test that a regular string is parsed. """
@@ -710,6 +711,26 @@ def testNestedAtomicString(self):
             '*to*</span> *test*</span> *with*</p></div>'
         )
 
+    def testInlineProcessorDoesntCrashWithAtomicString(self):
+        """ Test that an `AtomicString` returned from a Pattern doesn't cause a crash. """
+        tree = etree.Element('div')
+        p = etree.SubElement(tree, 'p')
+        p.text = 'a marker c'
+        self.md.inlinePatterns.register(
+            _InlineProcessorThatReturnsAtomicString(r'marker', self.md), 'test', 100
+        )
+        new = self.inlineprocessor.run(tree)
+        self.assertEqual(
+            markdown.serializers.to_html_string(new),
+            '<div><p>a <b>atomic</b> c</p></div>'
+        )
+
+
+class _InlineProcessorThatReturnsAtomicString(inlinepatterns.InlineProcessor):
+    """ Return a simple text of `group(1)` of a Pattern. """
+    def handleMatch(self, m, data):
+        return markdown.util.AtomicString('<b>atomic</b>'), m.start(0), m.end(0)
+
 
 class TestConfigParsing(unittest.TestCase):
     def assertParses(self, value, result):