diff --git a/mysql-test/suite/innodb/r/instant_alter_charset.result b/mysql-test/suite/innodb/r/instant_alter_charset.result
index 1f5f61f9451..82d6ce899c0 100644
--- a/mysql-test/suite/innodb/r/instant_alter_charset.result
+++ b/mysql-test/suite/innodb/r/instant_alter_charset.result
@@ -1820,3 +1820,10 @@ HEX(a)
 61
 62
 DROP TABLE t1;
+#
+# MDEV-19524 Server crashes in Bitmap<64u>::is_clear_all / Field_longstr::csinfo_change_allows_instant_alter
+#
+CREATE TABLE t1 (a VARCHAR(1), UNIQUE(a)) ENGINE=InnoDB;
+ALTER TABLE t1 MODIFY a INT, ADD b INT, ADD UNIQUE (b), ALGORITHM=INSTANT;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Cannot change column type. Try ALGORITHM=COPY
+DROP TABLE t1;
diff --git a/mysql-test/suite/innodb/t/instant_alter_charset.test b/mysql-test/suite/innodb/t/instant_alter_charset.test
index 37801b9414d..8c2d6112c8c 100644
--- a/mysql-test/suite/innodb/t/instant_alter_charset.test
+++ b/mysql-test/suite/innodb/t/instant_alter_charset.test
@@ -1,5 +1,4 @@
 --source include/innodb_row_format.inc
-#--source include/innodb_page_size.inc
 
 --let $row_format= `SELECT @@GLOBAL.innodb_default_row_format`
 set names utf8;
@@ -593,3 +592,15 @@ ALTER TABLE t1 ALGORITHM=INSTANT, MODIFY a VARCHAR(10) CHARACTER SET latin1 COLL
 ALTER IGNORE TABLE t1 MODIFY a VARCHAR(10) CHARACTER SET latin1 COLLATE latin1_general_ci;
 SELECT HEX(a) FROM t1;
 DROP TABLE t1;
+
+
+
+--echo #
+--echo # MDEV-19524 Server crashes in Bitmap<64u>::is_clear_all / Field_longstr::csinfo_change_allows_instant_alter
+--echo #
+
+CREATE TABLE t1 (a VARCHAR(1), UNIQUE(a)) ENGINE=InnoDB;
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+ALTER TABLE t1 MODIFY a INT, ADD b INT, ADD UNIQUE (b), ALGORITHM=INSTANT;
+DROP TABLE t1;
+
diff --git a/sql/sql_table.cc b/sql/sql_table.cc
index df5dea0bfad..7c1672ace93 100644
--- a/sql/sql_table.cc
+++ b/sql/sql_table.cc
@@ -6540,8 +6540,16 @@ Compare_keys compare_keys_but_name(const KEY *table_key, const KEY *new_key,
                            *new_part= new_key->key_part;
        key_part < end; key_part++, new_part++)
   {
+    /*
+      For prefix keys KEY_PART_INFO::field points to cloned Field
+      object with adjusted length. So below we have to check field
+      indexes instead of simply comparing pointers to Field objects.
+    */
     Create_field *new_field= alter_info->create_list.elem(new_part->fieldnr);
-    const Field *old_field= table->field[key_part->fieldnr - 1];
+    if (!new_field->field ||
+        new_field->field->field_index != key_part->fieldnr - 1)
+      return Compare_keys::NotEqual;
+
     /*
       If there is a change in index length due to column expansion
       like varchar(X) changed to varchar(X + N) and has a compatible
@@ -6551,6 +6559,7 @@ Compare_keys compare_keys_but_name(const KEY *table_key, const KEY *new_key,
       Key definition has changed if we are using a different field or
       if the user key part length is different.
     */
+    const Field *old_field= table->field[key_part->fieldnr - 1];
     auto old_field_len= old_field->pack_length();
 
     if (old_field->type() == MYSQL_TYPE_VARCHAR)
@@ -6568,15 +6577,6 @@ Compare_keys compare_keys_but_name(const KEY *table_key, const KEY *new_key,
     }
     else if (key_part->length != new_part->length)
       return Compare_keys::NotEqual;
-
-    /*
-      For prefix keys KEY_PART_INFO::field points to cloned Field
-      object with adjusted length. So below we have to check field
-      indexes instead of simply comparing pointers to Field objects.
-    */
-    if (!new_field->field ||
-        new_field->field->field_index != key_part->fieldnr - 1)
-      return Compare_keys::NotEqual;
   }
 
   /*