diff --git a/mysql-test/main/ctype_utf16.result b/mysql-test/main/ctype_utf16.result
index 71c7ee77a1d..5df3dbea274 100644
--- a/mysql-test/main/ctype_utf16.result
+++ b/mysql-test/main/ctype_utf16.result
@@ -2910,5 +2910,15 @@ CAST(_utf16 0x0061D83DDE0E0062 AS INT)
 Warnings:
 Warning	1292	Truncated incorrect INTEGER value: 'a?b'
 #
+# MDEV-23138 Odd behavior of character_set variables set to utf16 (when allowed)
+#
+SET character_set_connection=utf16;
+SET tx_isolation= 'READ-COMMITTED';
+SELECT @@tx_isolation;
+@@tx_isolation
+READ-COMMITTED
+SET tx_isolation=DEFAULT;
+SET NAMES utf8mb3;
+#
 # End of 10.5 tests
 #
diff --git a/mysql-test/main/ctype_utf16.test b/mysql-test/main/ctype_utf16.test
index 9f23f02b0fa..560b48e88f8 100644
--- a/mysql-test/main/ctype_utf16.test
+++ b/mysql-test/main/ctype_utf16.test
@@ -1029,6 +1029,15 @@ SET NAMES utf8;
 # surrogate pairs is replaced to a single question mark.
 SELECT CAST(_utf16 0x0061D83DDE0E0062 AS INT);
 
+--echo #
+--echo # MDEV-23138 Odd behavior of character_set variables set to utf16 (when allowed)
+--echo #
+
+SET character_set_connection=utf16;
+SET tx_isolation= 'READ-COMMITTED';
+SELECT @@tx_isolation;
+SET tx_isolation=DEFAULT;
+SET NAMES utf8mb3;
 
 --echo #
 --echo # End of 10.5 tests
diff --git a/sql/sys_vars.inl b/sql/sys_vars.inl
index 0c106777a3e..b678bbbd1cf 100644
--- a/sql/sys_vars.inl
+++ b/sql/sys_vars.inl
@@ -334,7 +334,11 @@ public:
 
     if (var->value->result_type() == STRING_RESULT)
     {
-      if (!(res=var->value->val_str(&str)))
+      /*
+        Convert from the expression character set to ascii.
+        This is OK, as typelib values cannot have non-ascii characters.
+      */
+      if (!(res= var->value->val_str_ascii(&str)))
         return true;
       else
       if (!(var->save_result.ulonglong_value=