diff --git a/Lib/idlelib/iomenu.py b/Lib/idlelib/iomenu.py
index 74ebefd..8bb2fa6 100644
--- a/Lib/idlelib/iomenu.py
+++ b/Lib/idlelib/iomenu.py
@@ -155,6 +155,17 @@
                                    parent=self.text)
             return False
 
+        if not isinstance(eol_convention, str):
+            # If the file does not contain line separators, it is None.
+            # If the file contains mixed line separators, it is a tuple.
+            if eol_convention is not None:
+                tkMessageBox.showwarning("Mixed Newlines",
+                                         "Mixed newlines detected.\n"
+                                         "The file will be changed on save.",
+                                         parent=self.text)
+                converted = True
+            eol_convention = os.linesep  # default
+
         self.text.delete("1.0", "end")
         self.set_filename(None)
         self.fileencoding = fileencoding
diff --git a/Misc/NEWS.d/next/IDLE/2020-07-24-17-49-58.bpo-41373.YQIPu_.rst b/Misc/NEWS.d/next/IDLE/2020-07-24-17-49-58.bpo-41373.YQIPu_.rst
new file mode 100644
index 0000000..b50a72fe
--- /dev/null
+++ b/Misc/NEWS.d/next/IDLE/2020-07-24-17-49-58.bpo-41373.YQIPu_.rst
@@ -0,0 +1,3 @@
+Save files loaded with no line ending, as when blank, or different line
+endings, by setting its line ending to the system default. Fix regression in
+3.8.4 and 3.9.0b4.