diff --git a/mysql-test/r/alter_table.result b/mysql-test/r/alter_table.result
index f0539559dc7..f714e6646de 100644
--- a/mysql-test/r/alter_table.result
+++ b/mysql-test/r/alter_table.result
@@ -1907,3 +1907,15 @@ t1	CREATE TABLE `t1` (
   `b` bigint(20) DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 DROP TABLE t1;
+create table t1 (i int);
+alter table t1 add unique index if not exists idx(i);
+alter table t1 add unique index if not exists idx(i);
+Warnings:
+Note	1061	Duplicate key name 'idx'
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `i` int(11) DEFAULT NULL,
+  UNIQUE KEY `idx` (`i`)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+DROP TABLE t1;
diff --git a/mysql-test/t/alter_table.test b/mysql-test/t/alter_table.test
index 247a79e2bda..2053e2d9d59 100644
--- a/mysql-test/t/alter_table.test
+++ b/mysql-test/t/alter_table.test
@@ -1651,3 +1651,12 @@ alter table t1 change column if exists a b bigint;
 show create table t1;
 DROP TABLE t1;
 
+#
+# MDEV-4437 ALTER TABLE .. ADD UNIQUE INDEX IF NOT EXISTS causes syntax error.
+#
+
+create table t1 (i int);
+alter table t1 add unique index if not exists idx(i);
+alter table t1 add unique index if not exists idx(i);
+show create table t1;
+DROP TABLE t1;
diff --git a/sql/sql_yacc.yy b/sql/sql_yacc.yy
index 52f54687266..7f39720ab56 100644
--- a/sql/sql_yacc.yy
+++ b/sql/sql_yacc.yy
@@ -955,10 +955,10 @@ bool my_yyoverflow(short **a, YYSTYPE **b, ulong *yystacksize);
 %parse-param { THD *thd }
 %lex-param { THD *thd }
 /*
-  Currently there are 164 shift/reduce conflicts.
+  Currently there are 163 shift/reduce conflicts.
   We should not introduce new conflicts any more.
 */
-%expect 164
+%expect 163
 
 /*
    Comments for TOKENS.
@@ -5900,7 +5900,7 @@ key_def:
             if (add_create_index (Lex, $1, $3))
               MYSQL_YYABORT;
           }
-        | opt_constraint constraint_key_type opt_ident key_alg
+        | opt_constraint constraint_key_type opt_if_not_exists_ident key_alg
           '(' key_list ')'
           { Lex->option_list= NULL; }
           normal_key_options