diff --git a/src/charset.c b/src/charset.c
index 29c587b..cdb9e48 100644
--- a/src/charset.c
+++ b/src/charset.c
@@ -960,7 +960,7 @@
 }
 
 /*
- * return TRUE if 'c' is a printable character
+ * Return TRUE if 'c' is a printable character.
  * Assume characters above 0x100 are printable (multi-byte), except for
  * Unicode.
  */
@@ -1717,7 +1717,7 @@
 {
     if (c <= '@')
 	return c;
-    if (c >= 0x80)
+    if (c >= 0x80 || !(cmp_flags & CMP_KEEPASCII))
     {
 	if (enc_utf8)
 	    return utf_toupper(c);
@@ -1741,7 +1741,7 @@
 {
     if (c <= '@')
 	return c;
-    if (c >= 0x80)
+    if (c >= 0x80 || !(cmp_flags & CMP_KEEPASCII))
     {
 	if (enc_utf8)
 	    return utf_tolower(c);
diff --git a/src/testdir/test_normal.vim b/src/testdir/test_normal.vim
index 8fce6e1..149afa9 100644
--- a/src/testdir/test_normal.vim
+++ b/src/testdir/test_normal.vim
@@ -1603,6 +1603,26 @@
   norm! V~
   call assert_equal('THIS IS A simple test: äüöss', getline('.'))
 
+  " Turkish ASCII turns to multi-byte.
+  try
+    lang tr_TR.UTF-8
+    set casemap=
+    call setline(1, 'iI')
+    1normal gUU
+    call assert_equal("\u0130I", getline(1))
+    call assert_equal("\u0130I", toupper("iI"))
+
+    call setline(1, 'iI')
+    1normal guu
+    call assert_equal("i\u0131", getline(1))
+    call assert_equal("i\u0131", tolower("iI"))
+
+    lang en_US.UTF-8
+  catch /E197:/
+    " can't use Turkish locale
+    throw 'Skipped: Turkish locale not available'
+  endtry
+
   " clean up
   bw!
 endfunc
diff --git a/src/version.c b/src/version.c
index 2ea5708..e37178b 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    552,
+/**/
     551,
 /**/
     550,