diff --git a/mysql-test/r/order_by_zerolength-4285.result b/mysql-test/r/order_by_zerolength-4285.result
index f60ce7d90c7..e4c117b26af 100644
--- a/mysql-test/r/order_by_zerolength-4285.result
+++ b/mysql-test/r/order_by_zerolength-4285.result
@@ -24,3 +24,23 @@ Warning	1292	Truncated incorrect CHAR(0) value: '8'
 Warning	1292	Truncated incorrect CHAR(0) value: '9'
 Warning	1292	Truncated incorrect CHAR(0) value: '10'
 drop table t1;
+#
+# MDEV-17020: Assertion `length > 0' failed in ptr_compare upon ORDER BY with bad conversion
+#
+set @save_sql_mode= @@sql_mode;
+SET @@sql_mode= '';
+CREATE TABLE t1 (pk INT PRIMARY KEY);
+INSERT INTO t1 VALUES (1),(2);
+explain
+SELECT * FROM t1 ORDER BY 'foo', CONVERT(pk, CHAR(0)) LIMIT 2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	2	Using index; Using filesort
+SELECT * FROM t1 ORDER BY 'foo', Cast(pk as CHAR(0)) LIMIT 2;
+pk
+1
+2
+Warnings:
+Warning	1292	Truncated incorrect CHAR(0) value: '1'
+Warning	1292	Truncated incorrect CHAR(0) value: '2'
+set @@sql_mode= @save_sql_mode;
+drop table t1;
diff --git a/mysql-test/t/order_by_zerolength-4285.test b/mysql-test/t/order_by_zerolength-4285.test
index 2fb58edd36d..9533f2cc62e 100644
--- a/mysql-test/t/order_by_zerolength-4285.test
+++ b/mysql-test/t/order_by_zerolength-4285.test
@@ -6,3 +6,16 @@ insert into t1 values (1),(2),(3),(4),(5),(6),(7),(8),(9),(10);
 select * from t1 order by now(), cast(pk as char(0));
 drop table t1;
 
+--echo #
+--echo # MDEV-17020: Assertion `length > 0' failed in ptr_compare upon ORDER BY with bad conversion
+--echo #
+
+set @save_sql_mode= @@sql_mode;
+SET @@sql_mode= '';
+CREATE TABLE t1 (pk INT PRIMARY KEY);
+INSERT INTO t1 VALUES (1),(2);
+explain
+SELECT * FROM t1 ORDER BY 'foo', CONVERT(pk, CHAR(0)) LIMIT 2;
+SELECT * FROM t1 ORDER BY 'foo', Cast(pk as CHAR(0)) LIMIT 2;
+set @@sql_mode= @save_sql_mode;
+drop table t1;
diff --git a/mysys/ptr_cmp.c b/mysys/ptr_cmp.c
index 6e373e98972..c9306dff14f 100644
--- a/mysys/ptr_cmp.c
+++ b/mysys/ptr_cmp.c
@@ -52,6 +52,11 @@ static int ptr_compare_0(size_t *compare_length, uchar **a, uchar **b);
 static int ptr_compare_1(size_t *compare_length, uchar **a, uchar **b);
 static int ptr_compare_2(size_t *compare_length, uchar **a, uchar **b);
 static int ptr_compare_3(size_t *compare_length, uchar **a, uchar **b);
+static int degenerate_compare_func(size_t *compare_length, uchar **a, uchar **b)
+{
+  DBUG_ASSERT(*compare_length == 0);
+  return 0;
+}
 #endif	/* __sun */
 
 	/* Get a pointer to a optimal byte-compare function for a given size */
@@ -64,6 +69,8 @@ qsort2_cmp get_ptr_compare (size_t size __attribute__((unused)))
 #else
 qsort2_cmp get_ptr_compare (size_t size)
 {
+  if (size == 0)
+    return (qsort2_cmp) degenerate_compare_func;
   if (size < 4)
     return (qsort2_cmp) ptr_compare;
   switch (size & 3) {