diff --git a/mysql-test/r/alter_table.result b/mysql-test/r/alter_table.result
index d1df0aced63..5a2b48219af 100644
--- a/mysql-test/r/alter_table.result
+++ b/mysql-test/r/alter_table.result
@@ -1893,3 +1893,9 @@ DROP TABLE ti3;
 CREATE TABLE tm1(i INT DEFAULT 1) engine=MyISAM;
 ALTER TABLE tm1 ADD INDEX ii1(i), ALTER COLUMN i DROP DEFAULT;
 DROP TABLE tm1;
+create table if not exists t1 (i int);
+alter table t1 add key (i);
+alter table t1 add key if not exists (i);
+Warnings:
+Note	1061	Duplicate key name 'i'
+DROP TABLE t1;
diff --git a/mysql-test/t/alter_table.test b/mysql-test/t/alter_table.test
index f40f8c11fac..b0b017f2b70 100644
--- a/mysql-test/t/alter_table.test
+++ b/mysql-test/t/alter_table.test
@@ -1634,3 +1634,12 @@ DROP TABLE ti3;
 CREATE TABLE tm1(i INT DEFAULT 1) engine=MyISAM;
 ALTER TABLE tm1 ADD INDEX ii1(i), ALTER COLUMN i DROP DEFAULT;
 DROP TABLE tm1;
+
+#
+# MDEV-4435 Server crashes in my_strcasecmp_utf8 on ADD KEY IF NOT EXISTS with implicit name when the key exists.
+#
+create table if not exists t1 (i int);
+alter table t1 add key (i);
+alter table t1 add key if not exists (i);
+DROP TABLE t1;
+
diff --git a/sql/sql_table.cc b/sql/sql_table.cc
index 887aef643d2..97d33055455 100644
--- a/sql/sql_table.cc
+++ b/sql/sql_table.cc
@@ -5389,17 +5389,29 @@ handle_if_exists_options(THD *thd, TABLE *table, Alter_info *alter_info)
     Key *key;
     List_iterator<Key> key_it(alter_info->key_list);
     uint n_key;
+    const char *keyname;
     while ((key=key_it++))
     {
       if (!key->create_if_not_exists)
         continue;
+      /* If the name of the key is not specified,     */
+      /* let us check the name of the first key part. */
+      if ((keyname= key->name.str) == NULL)
+      {
+        List_iterator<Key_part_spec> part_it(key->columns);
+        Key_part_spec *kp;
+        if ((kp= part_it++))
+          keyname= kp->field_name.str;
+        if (keyname == NULL)
+          continue;
+      }
       for (n_key=0; n_key < table->s->keys; n_key++)
       {
         if (my_strcasecmp(system_charset_info,
-              key->name.str, table->key_info[n_key].name) == 0)
+              keyname, table->key_info[n_key].name) == 0)
         {
           push_warning_printf(thd, Sql_condition::WARN_LEVEL_NOTE,
-              ER_DUP_KEYNAME, ER(ER_DUP_KEYNAME), key->name.str);
+              ER_DUP_KEYNAME, ER(ER_DUP_KEYNAME), keyname);
           key_it.remove();
           if (key->type == Key::FOREIGN_KEY)
           {
@@ -7924,7 +7936,8 @@ bool mysql_alter_table(THD *thd,char *new_db, char *new_name,
   if (alter_info->flags == 0)
   {
     my_snprintf(alter_ctx.tmp_name, sizeof(alter_ctx.tmp_name),
-                ER(ER_INSERT_INFO), 0L, 0L, 0L);
+                ER(ER_INSERT_INFO), 0L, 0L,
+                thd->get_stmt_da()->current_statement_warn_count());
     my_ok(thd, 0L, 0L, alter_ctx.tmp_name);
     DBUG_RETURN(false);
   }