diff --git a/mysql-test/r/type_bit.result b/mysql-test/r/type_bit.result
index 2a83d9b4c62..63dec0297d0 100644
--- a/mysql-test/r/type_bit.result
+++ b/mysql-test/r/type_bit.result
@@ -723,4 +723,30 @@ CREATE TABLE IF NOT EXISTS t1 (
 f1 bit(2) NOT NULL default b''
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci;
 ERROR 42000: Invalid default value for 'f1'
+create table t1bit7 (a1 bit(7) not null) engine=MyISAM;
+create table t2bit7 (b1 bit(7)) engine=MyISAM;
+insert into t1bit7 values (b'1100000');
+insert into t1bit7 values (b'1100001');
+insert into t1bit7 values (b'1100010');
+insert into t2bit7 values (b'1100001');
+insert into t2bit7 values (b'1100010');
+insert into t2bit7 values (b'1100110');
+select bin(a1) from t1bit7, t2bit7 where t1bit7.a1=t2bit7.b1;
+bin(a1)
+1100001
+1100010
+drop table t1bit7, t2bit7;
+create table t1bit7 (a1 bit(15) not null) engine=MyISAM;
+create table t2bit7 (b1 bit(15)) engine=MyISAM;
+insert into t1bit7 values (b'110000011111111');
+insert into t1bit7 values (b'110000111111111');
+insert into t1bit7 values (b'110001011111111');
+insert into t2bit7 values (b'110000111111111');
+insert into t2bit7 values (b'110001011111111');
+insert into t2bit7 values (b'110011011111111');
+select bin(a1) from t1bit7, t2bit7 where t1bit7.a1=t2bit7.b1;
+bin(a1)
+110000111111111
+110001011111111
+drop table t1bit7, t2bit7;
 End of 5.0 tests
diff --git a/mysql-test/t/type_bit.test b/mysql-test/t/type_bit.test
index 81dca17f112..bdc678688f1 100644
--- a/mysql-test/t/type_bit.test
+++ b/mysql-test/t/type_bit.test
@@ -367,4 +367,34 @@ CREATE TABLE IF NOT EXISTS t1 (
 f1 bit(2) NOT NULL default b''
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci;
 
+
+#
+# Bug#31399 Wrong query result when doing join buffering over BIT fields 
+#
+create table t1bit7 (a1 bit(7) not null) engine=MyISAM; 
+create table t2bit7 (b1 bit(7)) engine=MyISAM; 
+
+insert into t1bit7 values (b'1100000'); 
+insert into t1bit7 values (b'1100001'); 
+insert into t1bit7 values (b'1100010'); 
+insert into t2bit7 values (b'1100001'); 
+insert into t2bit7 values (b'1100010'); 
+insert into t2bit7 values (b'1100110'); 
+
+select bin(a1) from t1bit7, t2bit7 where t1bit7.a1=t2bit7.b1; 
+drop table t1bit7, t2bit7; 
+
+create table t1bit7 (a1 bit(15) not null) engine=MyISAM; 
+create table t2bit7 (b1 bit(15)) engine=MyISAM; 
+
+insert into t1bit7 values (b'110000011111111'); 
+insert into t1bit7 values (b'110000111111111'); 
+insert into t1bit7 values (b'110001011111111'); 
+insert into t2bit7 values (b'110000111111111'); 
+insert into t2bit7 values (b'110001011111111'); 
+insert into t2bit7 values (b'110011011111111'); 
+
+select bin(a1) from t1bit7, t2bit7 where t1bit7.a1=t2bit7.b1; 
+drop table t1bit7, t2bit7; 
+
 --echo End of 5.0 tests
diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index 2ac33c4e07f..b080fff8725 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -13233,6 +13233,7 @@ join_init_cache(THD *thd,JOIN_TAB *tables,uint table_count)
   length=0;
   for (i=0 ; i < table_count ; i++)
   {
+    bool have_bit_fields= FALSE;
     uint null_fields=0,used_fields;
 
     Field **f_ptr,*field;
@@ -13247,13 +13248,16 @@ join_init_cache(THD *thd,JOIN_TAB *tables,uint table_count)
 	length+=field->fill_cache_field(copy);
 	if (copy->blob_field)
 	  (*blob_ptr++)=copy;
-	if (field->maybe_null())
+	if (field->real_maybe_null())
 	  null_fields++;
+        if (field->type() == MYSQL_TYPE_BIT &&
+            ((Field_bit*)field)->bit_len)
+          have_bit_fields= TRUE;    
 	copy++;
       }
     }
     /* Copy null bits from table */
-    if (null_fields && tables[i].table->s->null_fields)
+    if (null_fields || have_bit_fields)
     {						/* must copy null bits */
       copy->str=(char*) tables[i].table->null_flags;
       copy->length= tables[i].table->s->null_bytes;