diff --git a/sql/item_strfunc.cc b/sql/item_strfunc.cc
index b7224117a24..e194150002b 100644
--- a/sql/item_strfunc.cc
+++ b/sql/item_strfunc.cc
@@ -1433,11 +1433,20 @@ String *Item_str_conv::val_str(String *str) {
     } else
       res = copy_if_not_alloced(str, res, res->length());
 
+    orig_res.copy(res->ptr(), res->length(), res->charset());
     len = converter(collation.collation, res->ptr(), res->length(), res->ptr(),
                     res->length());
+
+    if (len == 0) {
+      ++multiply;
+      res->swap(orig_res);
+      goto multiplyx;
+    }
+
     assert(len <= res->length());
     res->length(len);
   } else {
+multiplyx:
     size_t len = res->length() * multiply;
     tmp_value.alloc(len);
     tmp_value.set_charset(collation.collation);
diff --git a/sql/item_strfunc.h b/sql/item_strfunc.h
index 86d2b265f46..3f2e7bf4914 100644
--- a/sql/item_strfunc.h
+++ b/sql/item_strfunc.h
@@ -411,6 +411,7 @@ class Item_str_conv : public Item_str_func {
   uint multiply;
   my_charset_conv_case converter;
   String tmp_value;
+  String orig_res;
 
  public:
   Item_str_conv(const POS &pos, Item *item) : Item_str_func(pos, item) {}
diff --git a/strings/ctype-utf8.cc b/strings/ctype-utf8.cc
index 35876bec091..4d37bbae65c 100644
--- a/strings/ctype-utf8.cc
+++ b/strings/ctype-utf8.cc
@@ -7302,14 +7302,21 @@ static size_t my_caseup_utf8mb4(const CHARSET_INFO *cs, char *src,
   char *srcend = src + srclen, *dstend = dst + dstlen, *dst0 = dst;
   const MY_UNICASE_INFO *uni_plane = cs->caseinfo;
   assert(src != dst || cs->caseup_multiply == 1);
+  bool inplace = (src == dst);
 
   while ((src < srcend) &&
          (srcres = my_mb_wc_utf8mb4(&wc, (uchar *)src, (uchar *)srcend)) > 0) {
     my_toupper_utf8mb4(uni_plane, &wc);
-    if ((dstres = my_wc_mb_utf8mb4(cs, wc, (uchar *)dst, (uchar *)dstend)) <= 0)
+    if (((dstres = my_wc_mb_utf8mb4(cs, wc, (uchar *)dst, (uchar *)dstend)) <= 0) && !inplace)
       break;
-    src += srcres;
-    dst += dstres;
+    if (srcres == dstres || !inplace) {
+      src += srcres;
+      dst += dstres;
+    } else {
+      // srcres have to equal to dstres in in-place case conversion(multiply == 1), if not try bigger buffer.
+      // Not enough space to do in-place case conversion, which means some unicode character's bytes changed in utf8mb4.
+      return 0;
+    }
   }
   return (size_t)(dst - dst0);
 }
@@ -7388,14 +7395,19 @@ static size_t my_casedn_utf8mb4(const CHARSET_INFO *cs, char *src,
   char *srcend = src + srclen, *dstend = dst + dstlen, *dst0 = dst;
   const MY_UNICASE_INFO *uni_plane = cs->caseinfo;
   assert(src != dst || cs->casedn_multiply == 1);
+  bool inplace = (src == dst);
 
   while ((src < srcend) &&
          (srcres = my_mb_wc_utf8mb4(&wc, (uchar *)src, (uchar *)srcend)) > 0) {
     my_tolower_utf8mb4(uni_plane, &wc);
-    if ((dstres = my_wc_mb_utf8mb4(cs, wc, (uchar *)dst, (uchar *)dstend)) <= 0)
+    if (((dstres = my_wc_mb_utf8mb4(cs, wc, (uchar *)dst, (uchar *)dstend)) <= 0) && !inplace)
       break;
-    src += srcres;
-    dst += dstres;
+    if (srcres == dstres || !inplace) {
+      src += srcres;
+      dst += dstres;
+    } else {
+      return 0;
+    }
   }
   return (size_t)(dst - dst0);
 }