diff --git a/Lib/_pydatetime.py b/Lib/_pydatetime.py
index c1448374402de4..db4ea8d30c7064 100644
--- a/Lib/_pydatetime.py
+++ b/Lib/_pydatetime.py
@@ -358,7 +358,8 @@ def _find_isoformat_datetime_separator(dtstr):
 def _parse_isoformat_date(dtstr):
     # It is assumed that this is an ASCII-only string of lengths 7, 8 or 10,
     # see the comment on Modules/_datetimemodule.c:_find_isoformat_datetime_separator
-    assert len(dtstr) in (7, 8, 10)
+    if len(dtstr) not in (7, 8, 10):
+        raise ValueError("Invalid isoformat string")
     year = int(dtstr[0:4])
     has_sep = dtstr[4] == '-'
 
diff --git a/Lib/test/datetimetester.py b/Lib/test/datetimetester.py
index 1cbe78c1ecbfdc..e29f5e3ecb5fd4 100644
--- a/Lib/test/datetimetester.py
+++ b/Lib/test/datetimetester.py
@@ -3757,6 +3757,7 @@ def test_fromisoformat_fails_datetime(self):
             '2009-04-19T12:30:45+00:00:90', # Time zone field out from range
             '2009-04-19T12:30:45-00:90:00', # Time zone field out from range
             '2009-04-19T12:30:45-00:00:90', # Time zone field out from range
+            '2020-2020',                    # Ambiguous 9-char date portion
         ]
 
         for bad_str in bad_strs:
diff --git a/Misc/NEWS.d/next/Library/2026-06-24-10-46-35.gh-issue-152060.f2asrt.rst b/Misc/NEWS.d/next/Library/2026-06-24-10-46-35.gh-issue-152060.f2asrt.rst
new file mode 100644
index 00000000000000..6088a6b80829bb
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-06-24-10-46-35.gh-issue-152060.f2asrt.rst
@@ -0,0 +1,3 @@
+Fix :meth:`datetime.datetime.fromisoformat` raising :exc:`AssertionError`
+instead of :exc:`ValueError` for some malformed strings in the pure-Python
+implementation, matching the C implementation.