diff --git a/mysql-test/r/win.result b/mysql-test/r/win.result
index 6bd7f2c53d7..e7900bf8f09 100644
--- a/mysql-test/r/win.result
+++ b/mysql-test/r/win.result
@@ -423,3 +423,40 @@ from t1
 window w1 as (partition by c), w2 as (w1 order by pk);
 ERROR HY000: Unacceptable combination of window frame bound specifications
 drop table t0,t1;
+#
+# MDEV-9634: Window function produces incorrect value
+#
+create table t0 (a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t2 (part_id int, pk int, a int);
+insert into t2 select 
+if(a<5, 0, 1), a, if(a<5, NULL, 1) from t0;
+select * from t2;
+part_id	pk	a
+0	0	NULL
+0	1	NULL
+0	2	NULL
+0	3	NULL
+0	4	NULL
+1	5	1
+1	6	1
+1	7	1
+1	8	1
+1	9	1
+select 
+part_id, pk, a,
+count(a) over (partition by part_id order by pk
+rows between 1 preceding and 1 following) as CNT
+from t2;
+part_id	pk	a	CNT
+0	0	NULL	0
+0	1	NULL	0
+0	2	NULL	0
+0	3	NULL	0
+0	4	NULL	0
+1	5	1	2
+1	6	1	3
+1	7	1	3
+1	8	1	3
+1	9	1	2
+drop table t0, t2;
diff --git a/mysql-test/t/win.test b/mysql-test/t/win.test
index 5b2e908dd3f..b5dfa20c34b 100644
--- a/mysql-test/t/win.test
+++ b/mysql-test/t/win.test
@@ -284,7 +284,22 @@ window w1 as (partition by c), w2 as (w1 order by pk);
 
 drop table t0,t1;
 
+--echo #
+--echo # MDEV-9634: Window function produces incorrect value
+--echo #
 
+create table t0 (a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t2 (part_id int, pk int, a int);
+insert into t2 select 
+  if(a<5, 0, 1), a, if(a<5, NULL, 1) from t0;
+select * from t2;
 
+select 
+  part_id, pk, a,
+  count(a) over (partition by part_id order by pk
+                 rows between 1 preceding and 1 following) as CNT
+from t2;
 
+drop table t0, t2;
 
diff --git a/sql/item_sum.cc b/sql/item_sum.cc
index 91abea866b6..d9a92c7d216 100644
--- a/sql/item_sum.cc
+++ b/sql/item_sum.cc
@@ -1538,8 +1538,9 @@ bool Item_sum_count::add()
 
 void Item_sum_count::remove()
 {
-  /* TODO: Handle DECIMAL type */
   DBUG_ASSERT(aggr->Aggrtype() == Aggregator::SIMPLE_AGGREGATOR);
+  if (aggr->arg_is_null(false))
+    return;
   count--;
 }
 
diff --git a/sql/item_windowfunc.cc b/sql/item_windowfunc.cc
index 4bb8020b405..bf0d06b8837 100644
--- a/sql/item_windowfunc.cc
+++ b/sql/item_windowfunc.cc
@@ -65,6 +65,42 @@ Item_window_func::fix_fields(THD *thd, Item **ref)
 }
 
 
+/*
+  @detail
+    Window function evaluates its arguments when it is scanning the temporary
+    table in partition/order-by order. That is, arguments should be read from
+    the temporary table, not from the original base columns.
+
+    In order for this to work, we need to call "split_sum_func" for each
+    argument. The effect of the call is:
+     1. the argument is added into ref_pointer_array. This will cause the
+        argument to be saved in the temp.table
+     2. argument item is replaced with an Item_ref object. this object refers
+        the argument through the ref_pointer_array.
+
+    then, change_to_use_tmp_fields() will replace ref_pointer_array with an
+    array that points to the temp.table fields.
+    This way, when window_func attempts to evaluate its arguments, it will use
+    Item_ref objects which will read data from the temp.table.
+
+    Note: Before window functions, aggregate functions never needed to do such
+    transformations on their arguments. This is because grouping operation
+    does not need to read from the temp.table.
+    (Q: what happens when we first sort and then do grouping in a
+      group-after-group mode? dont group by items read from temp.table, then?)
+*/
+
+void Item_window_func::split_sum_func(THD *thd, Ref_ptr_array ref_pointer_array,
+                                      List<Item> &fields, uint flags)
+{
+  for (uint i=0; i < window_func->argument_count(); i++)
+  {
+    Item **p_item= &window_func->arguments()[i];
+    (*p_item)->split_sum_func2(thd, ref_pointer_array, fields, p_item, flags);
+  }
+}
+
+
 /*
   This must be called before advance_window() can be called.
 
diff --git a/sql/item_windowfunc.h b/sql/item_windowfunc.h
index e9a2d6936f2..d6e88e7c2d1 100644
--- a/sql/item_windowfunc.h
+++ b/sql/item_windowfunc.h
@@ -378,6 +378,8 @@ public:
                                          window_func->val_decimal(dec);
   }
 
+  void split_sum_func(THD *thd, Ref_ptr_array ref_pointer_array,
+                              List<Item> &fields, uint flags);
   void fix_length_and_dec()
   {
     window_func->fix_length_and_dec();
diff --git a/sql/sql_base.cc b/sql/sql_base.cc
index 980c15b18ac..20f602ce038 100644
--- a/sql/sql_base.cc
+++ b/sql/sql_base.cc
@@ -7966,8 +7966,12 @@ bool setup_fields(THD *thd, Ref_ptr_array ref_pointer_array,
       ref[0]= item;
       ref.pop_front();
     }
-    if (item->with_sum_func && item->type() != Item::SUM_FUNC_ITEM &&
-	sum_func_list)
+    /*
+      split_sum_func() must be called for Window Function items, see
+      Item_window_func::split_sum_func.
+    */
+    if ((item->with_sum_func && item->type() != Item::SUM_FUNC_ITEM &&
+	 sum_func_list) || item->type() == Item::WINDOW_FUNC_ITEM)
       item->split_sum_func(thd, ref_pointer_array, *sum_func_list,
                            SPLIT_SUM_SELECT);
     thd->lex->current_select->select_list_tables|= item->used_tables();