diff --git a/mysql-test/r/federated.result b/mysql-test/r/federated.result
index 49974615c68..2f81db25088 100644
--- a/mysql-test/r/federated.result
+++ b/mysql-test/r/federated.result
@@ -1689,6 +1689,22 @@ id	c1	c2
 9	abc	ppc
 drop table federated.t1, federated.t2;
 drop table federated.t1, federated.t2;
+create table t1 (id int not null auto_increment primary key, val int);
+create table t1
+(id int not null auto_increment primary key, val int) engine=federated
+connection='mysql://root@127.0.0.1:9308/test/t1';
+insert into t1 values (1,0),(2,0);
+update t1 set val = NULL where id = 1;
+select * from t1;
+id	val
+1	NULL
+2	0
+select * from t1;
+id	val
+1	NULL
+2	0
+drop table t1;
+drop table t1;
 DROP TABLE IF EXISTS federated.t1;
 DROP DATABASE IF EXISTS federated;
 DROP TABLE IF EXISTS federated.t1;
diff --git a/mysql-test/t/federated.test b/mysql-test/t/federated.test
index 780008cf13a..4af253dc326 100644
--- a/mysql-test/t/federated.test
+++ b/mysql-test/t/federated.test
@@ -1361,4 +1361,22 @@ drop table federated.t1, federated.t2;
 connection slave;
 drop table federated.t1, federated.t2;
 
+#
+# Bug #16494: Updates that set a column to NULL fail sometimes
+#
+connection slave;
+create table t1 (id int not null auto_increment primary key, val int);
+connection master;
+eval create table t1
+  (id int not null auto_increment primary key, val int) engine=federated
+  connection='mysql://root@127.0.0.1:$SLAVE_MYPORT/test/t1';
+insert into t1 values (1,0),(2,0);
+update t1 set val = NULL where id = 1;
+select * from t1;
+connection slave;
+select * from t1;
+drop table t1;
+connection master;
+drop table t1;
+
 source include/federated_cleanup.inc;
diff --git a/sql/ha_federated.cc b/sql/ha_federated.cc
index 11f676d9cf6..e2988df1619 100644
--- a/sql/ha_federated.cc
+++ b/sql/ha_federated.cc
@@ -1810,19 +1810,13 @@ int ha_federated::update_row(const byte *old_data, byte *new_data)
   /* 
     buffers for following strings
   */
-  char old_field_value_buffer[STRING_BUFFER_USUAL_SIZE];
-  char new_field_value_buffer[STRING_BUFFER_USUAL_SIZE];
+  char field_value_buffer[STRING_BUFFER_USUAL_SIZE];
   char update_buffer[FEDERATED_QUERY_BUFFER_SIZE];
   char where_buffer[FEDERATED_QUERY_BUFFER_SIZE];
 
-  /* stores the value to be replaced of the field were are updating */
-  String old_field_value(old_field_value_buffer,
-                         sizeof(old_field_value_buffer),
-                         &my_charset_bin);
-  /* stores the new value of the field */
-  String new_field_value(new_field_value_buffer,
-                         sizeof(new_field_value_buffer),
-                         &my_charset_bin);
+  /* Work area for field values */
+  String field_value(field_value_buffer, sizeof(field_value_buffer),
+                     &my_charset_bin);
   /* stores the update query */
   String update_string(update_buffer,
                        sizeof(update_buffer),
@@ -1835,8 +1829,7 @@ int ha_federated::update_row(const byte *old_data, byte *new_data)
   /* 
     set string lengths to 0 to avoid misc chars in string
   */
-  old_field_value.length(0);
-  new_field_value.length(0);
+  field_value.length(0);
   update_string.length(0);
   where_string.length(0);
 
@@ -1850,8 +1843,8 @@ int ha_federated::update_row(const byte *old_data, byte *new_data)
   In this loop, we want to match column names to values being inserted
   (while building INSERT statement).
 
-  Iterate through table->field (new data) and share->old_filed (old_data)
-  using the same index to created an SQL UPDATE statement, new data is
+  Iterate through table->field (new data) and share->old_field (old_data)
+  using the same index to create an SQL UPDATE statement. New data is
   used to create SET field=value and old data is used to create WHERE
   field=oldvalue
  */
@@ -1863,30 +1856,28 @@ int ha_federated::update_row(const byte *old_data, byte *new_data)
     update_string.append(FEDERATED_EQ);
 
     if ((*field)->is_null())
-      new_field_value.append(FEDERATED_NULL);
+      update_string.append(FEDERATED_NULL);
     else
     {
       /* otherwise = */
-      (*field)->val_str(&new_field_value);
-      (*field)->quote_data(&new_field_value);
-
-      if (!field_in_record_is_null(table, *field, (char*) old_data))
-        where_string.append(FEDERATED_EQ);
+      (*field)->val_str(&field_value);
+      (*field)->quote_data(&field_value);
+      update_string.append(field_value);
+      field_value.length(0);
     }
 
     if (field_in_record_is_null(table, *field, (char*) old_data))
       where_string.append(FEDERATED_ISNULL);
     else
     {
-      (*field)->val_str(&old_field_value,
+      where_string.append(FEDERATED_EQ);
+      (*field)->val_str(&field_value,
                         (char*) (old_data + (*field)->offset()));
-      (*field)->quote_data(&old_field_value);
-      where_string.append(old_field_value);
+      (*field)->quote_data(&field_value);
+      where_string.append(field_value);
+      field_value.length(0);
     }
 
-    update_string.append(new_field_value);
-    new_field_value.length(0);
-
     /*
       Only append conjunctions if we have another field in which
       to iterate
@@ -1896,7 +1887,6 @@ int ha_federated::update_row(const byte *old_data, byte *new_data)
       update_string.append(FEDERATED_COMMA);
       where_string.append(FEDERATED_AND);
     }
-    old_field_value.length(0);
   }
   update_string.append(FEDERATED_WHERE);
   update_string.append(where_string);