diff --git a/Docs/manual.texi b/Docs/manual.texi
index 232e0c0a2f1..5ded6e75cf1 100644
--- a/Docs/manual.texi
+++ b/Docs/manual.texi
@@ -49456,7 +49456,12 @@ Our TODO section contains what we plan to have in 4.0. @xref{TODO MySQL 4.0}.
 
 @itemize @bullet
 @item
-Fixed a bug in optimiser when a range specified makes index grouping impossible 
+Fixed a bug in optimiser with merge tables when non-uniques values are 
+used in summing up. 
+This consistently crashed MySQL.
+@item
+Fixed a bug in optimiser when a range specified makes index grouping impossible. 
+This consistently crashed MySQL.
 @item
 Fixed a rare bug when fulltext index is present and no tables are used
 @item
diff --git a/mysql-test/r/merge.result b/mysql-test/r/merge.result
index 9b72ff26350..1e657883253 100644
--- a/mysql-test/r/merge.result
+++ b/mysql-test/r/merge.result
@@ -462,3 +462,12 @@ a	b
 6	1
 6	2
 drop table if exists t6, t5, t4, t3, t2, t1;
+CREATE TABLE t1 (  a int(11) NOT NULL default '0',  b int(11) NOT NULL default '0',  PRIMARY KEY  (a,b)) TYPE=MyISAM;
+INSERT INTO t1 VALUES (1,1), (2,1);
+CREATE TABLE t2 (  a int(11) NOT NULL default '0',  b int(11) NOT NULL default '0',  PRIMARY KEY  (a,b)) TYPE=MyISAM;
+INSERT INTO t2 VALUES (1,2), (2,2);
+CREATE TABLE t (  a int(11) NOT NULL default '0',  b int(11) NOT NULL default '0',  KEY a (a,b)) TYPE=MRG_MyISAM UNION=(t1,t2);
+select max(b) from t where a = 2;
+max(b)
+NULL
+drop table if exists t,t1,t2;
diff --git a/mysql-test/t/merge.test b/mysql-test/t/merge.test
index 5bd78769a05..de26d7eb1d3 100644
--- a/mysql-test/t/merge.test
+++ b/mysql-test/t/merge.test
@@ -168,3 +168,10 @@ select * from t3 order by a,b;
 select * from t4 order by a,b;
 select * from t5 order by a,b;
 drop table if exists t6, t5, t4, t3, t2, t1;
+CREATE TABLE t1 (  a int(11) NOT NULL default '0',  b int(11) NOT NULL default '0',  PRIMARY KEY  (a,b)) TYPE=MyISAM;
+INSERT INTO t1 VALUES (1,1), (2,1);
+CREATE TABLE t2 (  a int(11) NOT NULL default '0',  b int(11) NOT NULL default '0',  PRIMARY KEY  (a,b)) TYPE=MyISAM;
+INSERT INTO t2 VALUES (1,2), (2,2);
+CREATE TABLE t (  a int(11) NOT NULL default '0',  b int(11) NOT NULL default '0',  KEY a (a,b)) TYPE=MRG_MyISAM UNION=(t1,t2);
+select max(b) from t where a = 2;
+drop table if exists t,t1,t2;
diff --git a/sql/opt_sum.cc b/sql/opt_sum.cc
index efb4c4916a5..7ffbb7ba8ea 100644
--- a/sql/opt_sum.cc
+++ b/sql/opt_sum.cc
@@ -152,11 +152,12 @@ int opt_sum_query(TABLE_LIST *tables, List<Item> &all_fields,COND *conds)
 	    error=table->file->index_last(table->record[0]) !=0;
 	  else
 	  {
-	    (void) table->file->index_read(table->record[0], key_buff,
+	    error= table->file->index_read(table->record[0], key_buff,
 					   ref.key_length,
 					   HA_READ_AFTER_KEY);
-	    error=table->file->index_prev(table->record[0]) ||
-	      key_cmp(table,key_buff,ref.key,ref.key_length);
+	    if (!error)
+	      error=table->file->index_prev(table->record[0]) ||
+		key_cmp(table,key_buff,ref.key,ref.key_length);
 	  }
 	  if (table->key_read)
 	  {