diff --git a/mysql-test/r/union.result b/mysql-test/r/union.result
index fbd4f8e11dc..7820cd1d6ff 100644
--- a/mysql-test/r/union.result
+++ b/mysql-test/r/union.result
@@ -1033,3 +1033,81 @@ a
 No
 aaa,bbb
 drop table t1,t2,t3,t4;
+create table t1 as
+(select _latin1'test') union
+(select _latin1'TEST') union
+(select _latin1'TeST');
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `test` char(4) NOT NULL default ''
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+select count(*) from t1;
+count(*)
+1
+drop table t1;
+create table t1 as
+(select _latin1'test' collate latin1_bin) union
+(select _latin1'TEST') union
+(select _latin1'TeST');
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `_latin1'test' collate latin1_bin` char(4) character set latin1 collate latin1_bin NOT NULL default ''
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+select count(*) from t1;
+count(*)
+3
+drop table t1;
+create table t1 as
+(select _latin1'test') union
+(select _latin1'TEST' collate latin1_bin) union
+(select _latin1'TeST');
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `test` char(4) character set latin1 collate latin1_bin NOT NULL default ''
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+select count(*) from t1;
+count(*)
+3
+drop table t1;
+create table t1 as
+(select _latin1'test') union
+(select _latin1'TEST') union
+(select _latin1'TeST' collate latin1_bin);
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `test` char(4) character set latin1 collate latin1_bin NOT NULL default ''
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+select count(*) from t1;
+count(*)
+3
+drop table t1;
+create table t2 (
+a char character set latin1 collate latin1_swedish_ci,
+b char character set latin1 collate latin1_bin);
+create table t1 as
+(select a from t2) union
+(select b from t2);
+ERROR HY000: Illegal mix of collations for operation 'UNION'
+create table t1 as
+(select a collate latin1_german1_ci from t2) union
+(select b from t2);
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a collate latin1_german1_ci` char(1) character set latin1 collate latin1_german1_ci default NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+drop table t1;
+create table t1 as
+(select a from t2) union
+(select b collate latin1_german1_ci from t2);
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` char(1) character set latin1 collate latin1_german1_ci default NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+drop table t1;
+drop table t2;
diff --git a/mysql-test/t/union.test b/mysql-test/t/union.test
index c5e72e85835..6e16a2b02aa 100644
--- a/mysql-test/t/union.test
+++ b/mysql-test/t/union.test
@@ -595,3 +595,58 @@ select a as a from t3 union select "1";
 select a as a from t4 union select a from t3;
 select a as a from t1 union select a from t4;
 drop table t1,t2,t3,t4;
+
+#
+# Bug #6139 UNION doesn't understand collate in the column of second select
+#
+create table t1 as
+(select _latin1'test') union
+(select _latin1'TEST') union
+(select _latin1'TeST');
+show create table t1;
+select count(*) from t1;
+drop table t1;
+
+create table t1 as
+(select _latin1'test' collate latin1_bin) union
+(select _latin1'TEST') union
+(select _latin1'TeST');
+show create table t1;
+select count(*) from t1;
+drop table t1;
+
+create table t1 as
+(select _latin1'test') union
+(select _latin1'TEST' collate latin1_bin) union
+(select _latin1'TeST');
+show create table t1;
+select count(*) from t1;
+drop table t1;
+
+create table t1 as
+(select _latin1'test') union
+(select _latin1'TEST') union
+(select _latin1'TeST' collate latin1_bin);
+show create table t1;
+select count(*) from t1;
+drop table t1;
+
+create table t2 (
+a char character set latin1 collate latin1_swedish_ci,
+b char character set latin1 collate latin1_bin);
+--error 1271
+create table t1 as
+(select a from t2) union
+(select b from t2);
+create table t1 as
+(select a collate latin1_german1_ci from t2) union
+(select b from t2);
+show create table t1;
+drop table t1;
+create table t1 as
+(select a from t2) union
+(select b collate latin1_german1_ci from t2);
+show create table t1;
+drop table t1;
+drop table t2;
+
diff --git a/sql/item.cc b/sql/item.cc
index 0366ea29485..304579d65a2 100644
--- a/sql/item.cc
+++ b/sql/item.cc
@@ -2558,8 +2558,8 @@ bool Item_type_holder::join_types(THD *thd, Item *item)
   if (use_new_field || use_expression_type ||
       (new_result_type != item_type) || (new_length > max_length) ||
       (!maybe_null && item->maybe_null) ||
-      (item_type == STRING_RESULT &&
-       !my_charset_same(collation.collation, item->collation.collation)))
+      (item_type == STRING_RESULT && 
+       collation.collation != item->collation.collation))
   {
     if (use_expression_type || item->type() != Item::FIELD_ITEM)
       field_example= 0;
diff --git a/sql/sql_union.cc b/sql/sql_union.cc
index b46cfc05538..6b5d27270c9 100644
--- a/sql/sql_union.cc
+++ b/sql/sql_union.cc
@@ -264,6 +264,23 @@ int st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
     }
   }
 
+  {
+    /*
+     Check that it was possible to aggregate all collations together.
+    */
+    List_iterator_fast<Item> tp(types);
+    Item *type;
+    while ((type= tp++))
+    {
+      if (type->result_type() == STRING_RESULT &&
+          type->collation.derivation == DERIVATION_NONE)
+      {
+        my_error(ER_CANT_AGGREGATE_NCOLLATIONS, MYF(0), "UNION");
+        goto err;
+      }
+    }
+  }
+
   // it is not single select
   if (first_select->next_select())
   {