diff --git a/mysql-test/r/win.result b/mysql-test/r/win.result
index bc017ea70a3..30650f29555 100644
--- a/mysql-test/r/win.result
+++ b/mysql-test/r/win.result
@@ -4198,5 +4198,37 @@ drop procedure sp7;
 drop view v1,v2;
 drop table t1;
 #
+# MDEV-17785: Window functions not working in ONLY_FULL_GROUP_BY mode
+#
+CREATE TABLE t1(a VARCHAR(10), b int);
+INSERT INTO t1 VALUES
+('Maths', 60),('Maths', 60),
+('Maths', 70),('Maths', 55),
+('Biology', 60), ('Biology', 70);
+SET @save_sql_mode= @@sql_mode;
+SET sql_mode = 'ONLY_FULL_GROUP_BY';
+SELECT
+RANK() OVER (PARTITION BY a ORDER BY b) AS rank,
+a, b FROM t1 ORDER BY a, b DESC;
+rank	a	b
+2	Biology	70
+1	Biology	60
+4	Maths	70
+2	Maths	60
+2	Maths	60
+1	Maths	55
+SET sql_mode= @save_sql_mode;
+DROP TABLE t1;
+CREATE TABLE t1(i int,j int);
+INSERT INTO t1 VALUES (1,1), (1,5),(1,4), (2,2),(2,5), (3,3),(4,4);
+INSERT INTO t1 VALUES (1,1), (1,5),(1,4), (2,2),(2,5), (3,3),(4,4);
+SELECT i, LAST_VALUE(COUNT(i)) OVER (PARTITION BY i ORDER BY j) FROM t1 GROUP BY i;
+i	LAST_VALUE(COUNT(i)) OVER (PARTITION BY i ORDER BY j)
+1	6
+2	4
+3	2
+4	2
+DROP TABLE t1;
+#
 # End of 10.2 tests
 #
diff --git a/mysql-test/suite/encryption/r/tempfiles_encrypted.result b/mysql-test/suite/encryption/r/tempfiles_encrypted.result
index f66a5fd0b25..aaba6bb044a 100644
--- a/mysql-test/suite/encryption/r/tempfiles_encrypted.result
+++ b/mysql-test/suite/encryption/r/tempfiles_encrypted.result
@@ -4204,6 +4204,38 @@ drop procedure sp7;
 drop view v1,v2;
 drop table t1;
 #
+# MDEV-17785: Window functions not working in ONLY_FULL_GROUP_BY mode
+#
+CREATE TABLE t1(a VARCHAR(10), b int);
+INSERT INTO t1 VALUES
+('Maths', 60),('Maths', 60),
+('Maths', 70),('Maths', 55),
+('Biology', 60), ('Biology', 70);
+SET @save_sql_mode= @@sql_mode;
+SET sql_mode = 'ONLY_FULL_GROUP_BY';
+SELECT
+RANK() OVER (PARTITION BY a ORDER BY b) AS rank,
+a, b FROM t1 ORDER BY a, b DESC;
+rank	a	b
+2	Biology	70
+1	Biology	60
+4	Maths	70
+2	Maths	60
+2	Maths	60
+1	Maths	55
+SET sql_mode= @save_sql_mode;
+DROP TABLE t1;
+CREATE TABLE t1(i int,j int);
+INSERT INTO t1 VALUES (1,1), (1,5),(1,4), (2,2),(2,5), (3,3),(4,4);
+INSERT INTO t1 VALUES (1,1), (1,5),(1,4), (2,2),(2,5), (3,3),(4,4);
+SELECT i, LAST_VALUE(COUNT(i)) OVER (PARTITION BY i ORDER BY j) FROM t1 GROUP BY i;
+i	LAST_VALUE(COUNT(i)) OVER (PARTITION BY i ORDER BY j)
+1	6
+2	4
+3	2
+4	2
+DROP TABLE t1;
+#
 # End of 10.2 tests
 #
 #
diff --git a/mysql-test/t/win.test b/mysql-test/t/win.test
index 72e789dff3f..126ed735c88 100644
--- a/mysql-test/t/win.test
+++ b/mysql-test/t/win.test
@@ -2703,6 +2703,33 @@ drop procedure sp7;
 drop view v1,v2;
 drop table t1;
 
+--echo #
+--echo # MDEV-17785: Window functions not working in ONLY_FULL_GROUP_BY mode
+--echo #
+
+CREATE TABLE t1(a VARCHAR(10), b int);
+
+INSERT INTO t1 VALUES
+('Maths', 60),('Maths', 60),
+('Maths', 70),('Maths', 55),
+('Biology', 60), ('Biology', 70);
+
+SET @save_sql_mode= @@sql_mode;
+SET sql_mode = 'ONLY_FULL_GROUP_BY';
+
+SELECT
+  RANK() OVER (PARTITION BY a ORDER BY b) AS rank,
+  a, b FROM t1 ORDER BY a, b DESC;
+
+SET sql_mode= @save_sql_mode;
+DROP TABLE t1;
+
+CREATE TABLE t1(i int,j int);
+INSERT INTO t1 VALUES (1,1), (1,5),(1,4), (2,2),(2,5), (3,3),(4,4);
+INSERT INTO t1 VALUES (1,1), (1,5),(1,4), (2,2),(2,5), (3,3),(4,4);
+SELECT i, LAST_VALUE(COUNT(i)) OVER (PARTITION BY i ORDER BY j) FROM t1 GROUP BY i;
+DROP TABLE t1;
+
 --echo #
 --echo # End of 10.2 tests
 --echo #
diff --git a/sql/item_windowfunc.cc b/sql/item_windowfunc.cc
index bb4a8a9f3af..03f99540771 100644
--- a/sql/item_windowfunc.cc
+++ b/sql/item_windowfunc.cc
@@ -93,6 +93,9 @@ Item_window_func::fix_fields(THD *thd, Item **ref)
     my_error(ER_NO_ORDER_LIST_IN_WINDOW_SPEC, MYF(0), window_func()->func_name());
     return true;
   }
+
+  window_func()->mark_as_window_func_sum_expr();
+
   /*
     TODO: why the last parameter is 'ref' in this call? What if window_func
     decides to substitute itself for something else and does *ref=.... ? 
diff --git a/sql/sql_yacc.yy b/sql/sql_yacc.yy
index a4b105862f3..6f3274aced5 100644
--- a/sql/sql_yacc.yy
+++ b/sql/sql_yacc.yy
@@ -10557,9 +10557,6 @@ window_func:
           simple_window_func
         |
           sum_expr
-          {
-            ((Item_sum *) $1)->mark_as_window_func_sum_expr();
-          }
         ;
 
 simple_window_func: