diff --git a/mysql-test/r/func_group.result b/mysql-test/r/func_group.result
index 69bce1c8bd8..b90eb2a4c0f 100644
--- a/mysql-test/r/func_group.result
+++ b/mysql-test/r/func_group.result
@@ -1746,4 +1746,15 @@ MAX(LENGTH(a))	LENGTH(MAX(a))	MIN(a)	MAX(a)	CONCAT(MIN(a))	CONCAT(MAX(a))
 20	20	18446668621106209655	18446668621106209655	18446668621106209655	18446668621106209655
 DROP TABLE t1;
 #
+# Bug #11766270  59343: YEAR(4): INCORRECT RESULT AND VALGRIND WARNINGS WITH MIN/MAX, UNION
+#
+CREATE TABLE t1(f1 YEAR(4));
+INSERT INTO t1 VALUES (0000),(2001);
+(SELECT MAX(f1) FROM t1) UNION (SELECT MAX(f1) FROM t1);
+Catalog	Database	Table	Table_alias	Column	Column_alias	Type	Length	Max length	Is_null	Flags	Decimals	Charsetnr
+def				MAX(f1)	MAX(f1)	13	4	4	Y	32864	0	63
+MAX(f1)
+2001
+DROP TABLE t1;
+#
 End of 5.1 tests
diff --git a/mysql-test/t/func_group.test b/mysql-test/t/func_group.test
index 600b46fcde6..177a1ca2471 100644
--- a/mysql-test/t/func_group.test
+++ b/mysql-test/t/func_group.test
@@ -1127,6 +1127,18 @@ INSERT INTO t1 VALUES (18446668621106209655);
 SELECT MAX(LENGTH(a)), LENGTH(MAX(a)), MIN(a), MAX(a), CONCAT(MIN(a)), CONCAT(MAX(a)) FROM t1;
 DROP TABLE t1;
 
+--echo #
+--echo # Bug #11766270  59343: YEAR(4): INCORRECT RESULT AND VALGRIND WARNINGS WITH MIN/MAX, UNION
+--echo #
+
+CREATE TABLE t1(f1 YEAR(4));
+INSERT INTO t1 VALUES (0000),(2001);
+--enable_metadata
+(SELECT MAX(f1) FROM t1) UNION (SELECT MAX(f1) FROM t1);
+--disable_metadata
+DROP TABLE t1;
+
+
 --echo #
 --echo End of 5.1 tests
 
diff --git a/sql/field.cc b/sql/field.cc
index 1ad5e408e07..3707c5b056f 100644
--- a/sql/field.cc
+++ b/sql/field.cc
@@ -5467,6 +5467,7 @@ double Field_year::val_real(void)
 longlong Field_year::val_int(void)
 {
   ASSERT_COLUMN_MARKED_FOR_READ;
+  DBUG_ASSERT(field_length == 2 || field_length == 4);
   int tmp= (int) ptr[0];
   if (field_length != 4)
     tmp%=100;					// Return last 2 char
@@ -5479,6 +5480,7 @@ longlong Field_year::val_int(void)
 String *Field_year::val_str(String *val_buffer,
 			    String *val_ptr __attribute__((unused)))
 {
+  DBUG_ASSERT(field_length < 5);
   val_buffer->alloc(5);
   val_buffer->length(field_length);
   char *to=(char*) val_buffer->ptr();
diff --git a/sql/item_sum.cc b/sql/item_sum.cc
index 2a8aea68f7a..c62738abac0 100644
--- a/sql/item_sum.cc
+++ b/sql/item_sum.cc
@@ -612,17 +612,13 @@ Item_sum_hybrid::fix_fields(THD *thd, Item **ref)
 
   switch (hybrid_type= item->result_type()) {
   case INT_RESULT:
-    max_length= 20;
-    break;
   case DECIMAL_RESULT:
+  case STRING_RESULT:
     max_length= item->max_length;
     break;
   case REAL_RESULT:
     max_length= float_length(decimals);
     break;
-  case STRING_RESULT:
-    max_length= item->max_length;
-    break;
   case ROW_RESULT:
   default:
     DBUG_ASSERT(0);