diff --git a/aider/coders/wholefile_coder.py b/aider/coders/wholefile_coder.py
index 48e3339e3..2ba813dae 100644
--- a/aider/coders/wholefile_coder.py
+++ b/aider/coders/wholefile_coder.py
@@ -44,7 +44,7 @@ class WholeFileCoder(Coder):
                     full_path = (Path(self.root) / fname).absolute()
 
                     if mode == "diff" and full_path.exists():
-                        orig_lines = full_path.read_text().splitlines()
+                        orig_lines = full_path.read_text().splitlines(keepends=True)
 
                         show_diff = diffs.diff_partial_update(
                             orig_lines,
@@ -97,7 +97,7 @@ class WholeFileCoder(Coder):
                 full_path = (Path(self.root) / fname).absolute()
 
                 if mode == "diff" and full_path.exists():
-                    orig_lines = full_path.read_text().splitlines()
+                    orig_lines = full_path.read_text().splitlines(keepends=True)
 
                     show_diff = diffs.diff_partial_update(
                         orig_lines,
diff --git a/aider/diffs.py b/aider/diffs.py
index a2a8c52f6..4608e8e18 100644
--- a/aider/diffs.py
+++ b/aider/diffs.py
@@ -33,6 +33,13 @@ def create_progress_bar(percentage):
     return bar
 
 
+def assert_newlines(lines):
+    if not lines:
+        return
+    for line in lines:
+        assert line and line[-1] == "\n", line
+
+
 def diff_partial_update(lines_orig, lines_updated, final=False, fname=None):
     """
     Given only the first part of an updated file, show the diff while
@@ -43,6 +50,9 @@ def diff_partial_update(lines_orig, lines_updated, final=False, fname=None):
     # dump(lines_orig)
     # dump(lines_updated)
 
+    assert_newlines(lines_orig)
+    assert_newlines(lines_orig)
+
     num_orig_lines = len(lines_orig)
 
     if final: