diff --git a/changelog/10840.improvement.rst b/changelog/10840.improvement.rst
new file mode 100644
index 000000000..485f46cec
--- /dev/null
+++ b/changelog/10840.improvement.rst
@@ -0,0 +1 @@
+pytest should no longer crash on AST with pathological position attributes.
diff --git a/src/_pytest/_code/code.py b/src/_pytest/_code/code.py
index e375fb70c..032b83beb 100644
--- a/src/_pytest/_code/code.py
+++ b/src/_pytest/_code/code.py
@@ -743,11 +743,13 @@ class FormattedExcinfo:
     ) -> List[str]:
         """Return formatted and marked up source lines."""
         lines = []
-        if source is None or line_index >= len(source.lines):
+        if source is not None and line_index < 0:
+            line_index += len(source)
+        if source is None or line_index >= len(source.lines) or line_index < 0:
+            # `line_index` could still be outside `range(len(source.lines))` if
+            # we're processing AST with pathological position attributes.
             source = Source("???")
             line_index = 0
-        if line_index < 0:
-            line_index += len(source)
         space_prefix = "    "
         if short:
             lines.append(space_prefix + source.lines[line_index].strip())