diff --git a/bin/pt-online-schema-change b/bin/pt-online-schema-change
index ac49730c..32729f74 100755
--- a/bin/pt-online-schema-change
+++ b/bin/pt-online-schema-change
@@ -2199,7 +2199,7 @@ sub parse {
    foreach my $key ( keys %$opts ) {
       PTDEBUG && _d('Finding value for', $key);
       $final_props{$key} = $given_props{$key};
-      if (   !defined $final_props{$key}
+      if ( !defined $final_props{$key}  
            && defined $prev->{$key} && $opts->{$key}->{copy} )
       {
          $final_props{$key} = $prev->{$key};
@@ -2316,7 +2316,6 @@ sub get_dbh {
       PrintError         => 0,
       ShowErrorStatement => 1,
       mysql_enable_utf8 => ($cxn_string =~ m/charset=utf8/i ? 1 : 0),
-      #      mysql_multi_statements => 1,
    };
    @{$defaults}{ keys %$opts } = values %$opts;
    if (delete $defaults->{L}) { # L for LOAD DATA LOCAL INFILE, our own extension
@@ -3444,7 +3443,6 @@ sub check_table {
       $self->{check_table_error} = $e;
       return 0;
    }
-
    if ( !$row->[0] || $row->[0] ne $tbl ) {
       PTDEBUG && _d('Table does not exist');
       return 0;
@@ -4988,9 +4986,6 @@ sub wait {
 
 sub _d {
    my ($package, undef, $line) = caller 0;
-   #   Backslash found where operator expected at ./pt-online-schema-change line 4933, near "@_ = map { (my $temp = $_) =~ s/\"
-   # (Might be a runaway multi-line // string starting on line 4889)
-
    @_ = map { (my $temp = $_) =~ s/\n/\n# /g; $temp; }
         map { defined $_ ? $_ : 'undef' }
         @_;
@@ -5821,23 +5816,26 @@ sub _find_best_index {
    my $tbl_struct = $tbl->{tbl_struct};
    my $indexes    = $tbl_struct->{keys};
 
+   my $best_index;
    my $want_index = $args{chunk_index};
    if ( $want_index ) {
       PTDEBUG && _d('User wants to use index', $want_index);
       if ( !exists $indexes->{$want_index} ) {
          PTDEBUG && _d('Cannot use user index because it does not exist');
          $want_index = undef;
+      } else {
+         $best_index = $want_index;
       }
    }
 
-   if ( !$want_index && $args{mysql_index} ) {
+   if ( !$best_index && !$want_index && $args{mysql_index} ) {
       PTDEBUG && _d('MySQL wants to use index', $args{mysql_index});
       $want_index = $args{mysql_index};
    }
 
-   my $best_index;
+
    my @possible_indexes;
-   if ( $want_index ) {
+   if ( !$best_index && $want_index ) {
       if ( $indexes->{$want_index}->{is_unique} ) {
          PTDEBUG && _d('Will use wanted index');
          $best_index = $want_index;
@@ -5847,7 +5845,8 @@ sub _find_best_index {
          push @possible_indexes, $want_index;
       }
    }
-   else {
+   
+   if (!$best_index) {
       PTDEBUG && _d('Auto-selecting best index');
       foreach my $index ( $tp->sort_indexes($tbl_struct) ) {
          if ( $index eq 'PRIMARY' || $indexes->{$index}->{is_unique} ) {
diff --git a/t/pt-online-schema-change/issue-1646713.t b/t/pt-online-schema-change/issue-1646713.t
index f077349f..3d716db8 100644
--- a/t/pt-online-schema-change/issue-1646713.t
+++ b/t/pt-online-schema-change/issue-1646713.t
@@ -57,7 +57,7 @@ sub start_thread {
    $dbh->do("UPDATE `test`.`o1` SET id=0 WHERE id=1");
    diag("Row updated");
 }
-my $thr = threads->create('start_thread', $dsn_opts, 3);
+my $thr = threads->create('start_thread', $dsn_opts, 1);
 $thr->detach();
 threads->yield();
 
@@ -72,7 +72,6 @@ $output = output(
       },
 );
 
-
 like(
       $output,
       qr/Successfully altered/s,
diff --git a/t/pt-online-schema-change/samples/slave_lag.sql b/t/pt-online-schema-change/samples/slave_lag.sql
index 5a95dfd3..55ab2a8a 100644
--- a/t/pt-online-schema-change/samples/slave_lag.sql
+++ b/t/pt-online-schema-change/samples/slave_lag.sql
@@ -37,7 +37,7 @@ DROP TABLE IF EXISTS `pt178`;
 /*!40101 SET character_set_client = utf8 */;
 CREATE TABLE `pt178` (
   `id` int(11) NOT NULL AUTO_INCREMENT,
-  f1 VARCHAR(30) NULL,
+  f1 VARCHAR(30) DEFAULT '',
   f2 BIGINT(11) DEFAULT 0,
   PRIMARY KEY(id)
 ) ENGINE=InnoDB;
diff --git a/t/pt-online-schema-change/slave_lag.t b/t/pt-online-schema-change/slave_lag.t
index 1e46aa10..2631e21d 100644
--- a/t/pt-online-schema-change/slave_lag.t
+++ b/t/pt-online-schema-change/slave_lag.t
@@ -42,15 +42,20 @@ $slave_dbh->do('STOP SLAVE');
 $slave_dbh->do('RESET SLAVE');
 $slave_dbh->do('START SLAVE');
 
-diag('Loading test data');
-$sb->load_file('master', "t/pt-online-schema-change/samples/slave_lag.sql");
-
 diag("Setting slave delay to $delay seconds");
 
 $slave_dbh->do('STOP SLAVE');
 $slave_dbh->do("CHANGE MASTER TO MASTER_DELAY=$delay");
 $slave_dbh->do('START SLAVE');
 
+diag('Loading test data');
+$sb->load_file('master', "t/pt-online-schema-change/samples/slave_lag.sql");
+
+# Run a full table scan query to ensure the slave is behind the master
+$master_dbh->do('RESET QUERY CACHE');
+$slave_dbh->do('RESET QUERY CACHE');
+$master_dbh->do('UPDATE `test`.`pt178` SET f2 = f2 + 1 WHERE f1 = ""'); 
+
 # This is the base test, ust to ensure that without using --check-slave-lag nor --skip-check-slave-lag
 # pt-online-schema-change will wait on the slave at port 12346
 
@@ -69,6 +74,11 @@ like(
 $args = "$master_dsn,D=test,t=pt178 --execute --chunk-size 1 --max-lag 5 --alter 'ENGINE=InnoDB' "
       . "--check-slave-lag h=127.1,P=12346,u=msandbox,p=msandbox,D=test,t=sbtest";
 
+# Run a full table scan query to ensure the slave is behind the master
+$master_dbh->do('RESET QUERY CACHE');
+$slave_dbh->do('RESET QUERY CACHE');
+$master_dbh->do('UPDATE `test`.`pt178` SET f2 = f2 + 1 WHERE f1 = ""'); 
+
 diag("Starting --check-slave-lag test. This is going to take some time due to the delay in the slave");
 $output = `$trunk/bin/pt-online-schema-change $args 2>&1`;
 
@@ -79,6 +89,11 @@ like(
 );
 
 # Repeat the test now using --skip-check-slave-lag
+# Run a full table scan query to ensure the slave is behind the master
+$master_dbh->do('RESET QUERY CACHE');
+$slave_dbh->do('RESET QUERY CACHE');
+$master_dbh->do('UPDATE `test`.`pt178` SET f2 = f2 + 1 WHERE f1 = ""'); 
+
 $args = "$master_dsn,D=test,t=pt178 --execute --chunk-size 1 --max-lag 5 --alter 'ENGINE=InnoDB' "
       . "--skip-check-slave-lag h=127.0.0.1,P=12346,u=msandbox,p=msandbox,D=test,t=sbtest";
 
@@ -91,6 +106,11 @@ unlike(
       "--skip-check-slave-lag is really skipping the slave",
 );
 
+diag("Setting slave delay to 0 seconds");
+$slave_dbh->do('STOP SLAVE');
+$slave_dbh->do('RESET SLAVE');
+$slave_dbh->do('START SLAVE');
+
 $master_dbh->do("DROP DATABASE IF EXISTS test");
 
 # #############################################################################