diff --git a/mysql-test/r/func_str.result b/mysql-test/r/func_str.result
index 464178124ecb7..b1862d46613af 100644
--- a/mysql-test/r/func_str.result
+++ b/mysql-test/r/func_str.result
@@ -4887,5 +4887,19 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 Warnings:
 Note	1003	select octet_length('a') AS `LENGTH('a')`,octet_length('a') AS `LENGTHB('a')`
 #
+# MDEV-13555 Assertion `!item->null_value' failed in Type_handler::Item_send_str
+#
+CREATE TABLE t1 (c VARCHAR(8));
+INSERT INTO t1 VALUES (NULL),('bar');
+SELECT LPAD( c, 0, '?' ) FROM t1;
+LPAD( c, 0, '?' )
+NULL
+
+SELECT RPAD( c, 0, '?' ) FROM t1;
+RPAD( c, 0, '?' )
+NULL
+
+DROP TABLE t1;
+#
 # End of 10.3 tests
 #
diff --git a/mysql-test/t/func_str.test b/mysql-test/t/func_str.test
index 2f7dd638445a3..a75330ab981e4 100644
--- a/mysql-test/t/func_str.test
+++ b/mysql-test/t/func_str.test
@@ -1889,6 +1889,18 @@ DROP TABLE  t1;
 SELECT LENGTHB('a'), LENGTHB(_utf8 0xC39F), LENGTHB(123), LENGTH(null);
 EXPLAIN EXTENDED SELECT LENGTH('a'), LENGTHB('a');
 
+
+--echo #
+--echo # MDEV-13555 Assertion `!item->null_value' failed in Type_handler::Item_send_str
+--echo #
+
+CREATE TABLE t1 (c VARCHAR(8));
+INSERT INTO t1 VALUES (NULL),('bar');
+SELECT LPAD( c, 0, '?' ) FROM t1;
+SELECT RPAD( c, 0, '?' ) FROM t1;
+DROP TABLE t1;
+
+
 --echo #
 --echo # End of 10.3 tests
 --echo #
diff --git a/sql/item_strfunc.cc b/sql/item_strfunc.cc
index 3b1dbf57fc4db..6138b67c02e04 100644
--- a/sql/item_strfunc.cc
+++ b/sql/item_strfunc.cc
@@ -3267,10 +3267,11 @@ String *Item_func_rpad::val_str(String *str)
       ((count < 0) && !args[1]->unsigned_flag))
     goto err;
 
+  null_value=0;
+
   if (count == 0)
     return make_empty_result();
 
-  null_value=0;
   /* Assumes that the maximum length of a String is < INT_MAX32. */
   /* Set here so that rest of code sees out-of-bound value as such. */
   if ((ulonglong) count > INT_MAX32)
@@ -3358,10 +3359,11 @@ String *Item_func_lpad::val_str(String *str)
       ((count < 0) && !args[1]->unsigned_flag))
     goto err;  
 
+  null_value=0;
+
   if (count == 0)
     return make_empty_result();
 
-  null_value=0;
   /* Assumes that the maximum length of a String is < INT_MAX32. */
   /* Set here so that rest of code sees out-of-bound value as such. */
   if ((ulonglong) count > INT_MAX32)