diff --git a/mysql-test/r/type_varchar.result b/mysql-test/r/type_varchar.result
index 3bd7fe6b175..fed03cd8d71 100644
--- a/mysql-test/r/type_varchar.result
+++ b/mysql-test/r/type_varchar.result
@@ -392,3 +392,26 @@ group by t1.b, t1.a;
 a	b	min(t1.b)
 22	NULL	NULL
 drop table t1, t2;
+create table t1 (f1 varchar(65500));
+create index index1 on t1(f1(10));
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `f1` varchar(65500) default NULL,
+  KEY `index1` (`f1`(10))
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+alter table t1 modify f1 varchar(255);
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `f1` varchar(255) default NULL,
+  KEY `index1` (`f1`(10))
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+alter table t1 modify f1 tinytext;
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `f1` tinytext,
+  KEY `index1` (`f1`(10))
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+drop table t1;
diff --git a/mysql-test/t/type_varchar.test b/mysql-test/t/type_varchar.test
index 2bffca6b889..1a3a93018a4 100644
--- a/mysql-test/t/type_varchar.test
+++ b/mysql-test/t/type_varchar.test
@@ -118,3 +118,15 @@ insert into t2 values (22), (22);
 select t1.a, t1.b, min(t1.b) from t1 inner join t2 ON t2.a = t1.a 
   group by t1.b, t1.a;
 drop table t1, t2;
+
+#
+# Bug #10543: convert varchar with index to text
+#
+create table t1 (f1 varchar(65500));
+create index index1 on t1(f1(10));
+show create table t1;
+alter table t1 modify f1 varchar(255);
+show create table t1;
+alter table t1 modify f1 tinytext;
+show create table t1;
+drop table t1;
diff --git a/sql/field.cc b/sql/field.cc
index 54ed4044de5..18f201243dd 100644
--- a/sql/field.cc
+++ b/sql/field.cc
@@ -982,6 +982,39 @@ Item_result Field::result_merge_type(enum_field_types field_type)
   Static help functions
 *****************************************************************************/
 
+
+/*
+  Check whether a field type can be partially indexed by a key
+
+  This is a static method, rather than a virtual function, because we need
+  to check the type of a non-Field in mysql_alter_table().
+
+  SYNOPSIS
+   type_can_have_key_part()
+   type                 field type
+
+  RETURN
+    TRUE  Type can have a prefixed key
+    FALSE Type can not have a prefixed key
+*/
+
+bool Field::type_can_have_key_part(enum enum_field_types type)
+{
+  switch (type) {
+  case MYSQL_TYPE_VARCHAR:
+  case MYSQL_TYPE_TINY_BLOB:
+  case MYSQL_TYPE_MEDIUM_BLOB:
+  case MYSQL_TYPE_LONG_BLOB:
+  case MYSQL_TYPE_BLOB:
+  case MYSQL_TYPE_VAR_STRING:
+  case MYSQL_TYPE_STRING:
+    return TRUE;
+  default:
+    return FALSE;
+  }
+}
+
+
 /*
   Numeric fields base class constructor
 */
diff --git a/sql/field.h b/sql/field.h
index f68a2327dff..8510c59c9f2 100644
--- a/sql/field.h
+++ b/sql/field.h
@@ -119,6 +119,7 @@ public:
   virtual Item_result result_type () const=0;
   virtual Item_result cmp_type () const { return result_type(); }
   virtual Item_result cast_to_int_type () const { return result_type(); }
+  static bool type_can_have_key_part(enum_field_types);
   static enum_field_types field_type_merge(enum_field_types, enum_field_types);
   static Item_result result_merge_type(enum_field_types);
   bool eq(Field *field)
diff --git a/sql/sql_table.cc b/sql/sql_table.cc
index 4ddef3fc653..2768d503751 100644
--- a/sql/sql_table.cc
+++ b/sql/sql_table.cc
@@ -3334,12 +3334,21 @@ bool mysql_alter_table(THD *thd,char *new_db, char *new_name,
 	continue;				// Field is removed
       uint key_part_length=key_part->length;
       if (cfield->field)			// Not new field
-      {						// Check if sub key
-	if (cfield->field->type() != FIELD_TYPE_BLOB &&
-	    (cfield->field->pack_length() == key_part_length ||
-	     cfield->length <= key_part_length /
-			       key_part->field->charset()->mbmaxlen))
-	  key_part_length=0;			// Use whole field
+      {
+        /*
+          If the field can't have only a part used in a key according to its
+          new type, or should not be used partially according to its
+          previous type, or the field length is less than the key part
+          length, unset the key part length.
+
+          BLOBs may have cfield->length == 0, which is why we test it before
+          checking whether cfield->length < key_part_length (in chars).
+         */
+        if (!Field::type_can_have_key_part(cfield->field->type()) ||
+            !Field::type_can_have_key_part(cfield->sql_type) ||
+	    (cfield->length && (cfield->length < key_part_length /
+                                key_part->field->charset()->mbmaxlen)))
+	  key_part_length= 0;			// Use whole field
       }
       key_part_length /= key_part->field->charset()->mbmaxlen;
       key_parts.push_back(new key_part_spec(cfield->field_name,