diff --git a/mysql-test/main/ctype_utf8.result b/mysql-test/main/ctype_utf8.result
index 670cd9242b94b..dd1c12e4a383f 100644
--- a/mysql-test/main/ctype_utf8.result
+++ b/mysql-test/main/ctype_utf8.result
@@ -1521,9 +1521,9 @@ id	a	b
 11	bbbbbb	60
 SELECT id, a, b FROM t1 WHERE a BETWEEN 'aaaa' AND 'bbbbbb';
 id	a	b
+6	bbbbbb	40
 8	aaaa	10
 9	aaaa	50
-6	bbbbbb	40
 11	bbbbbb	60
 SELECT id, a FROM t1 WHERE a='bbbbbb';
 id	a
diff --git a/mysql-test/main/ctype_utf8mb4.result b/mysql-test/main/ctype_utf8mb4.result
index c97ba3b6d5369..26a273906d2ba 100644
--- a/mysql-test/main/ctype_utf8mb4.result
+++ b/mysql-test/main/ctype_utf8mb4.result
@@ -1546,9 +1546,9 @@ id	a	b
 11	bbbbbb	60
 SELECT id, a, b FROM t1 WHERE a BETWEEN 'aaaa' AND 'bbbbbb';
 id	a	b
+6	bbbbbb	40
 8	aaaa	10
 9	aaaa	50
-6	bbbbbb	40
 11	bbbbbb	60
 SELECT id, a FROM t1 WHERE a='bbbbbb';
 id	a
diff --git a/mysql-test/main/range.result b/mysql-test/main/range.result
index 188bae82f1a82..e304836dc4edf 100644
--- a/mysql-test/main/range.result
+++ b/mysql-test/main/range.result
@@ -3873,3 +3873,26 @@ DROP TABLE lineitem;
 #
 # End of 11.0 tests
 #
+#
+# MDEV-39843: Prefix index causes incorrect range query result
+#
+SET NAMES utf8mb4;
+CREATE TABLE t0 (c0 LONGTEXT, KEY i0 (c0(1)));
+INSERT INTO t0(c0) VALUES ('꙰dcF'), ('kER');
+SELECT c0 FROM t0 WHERE 'C0' <= t0.c0;
+c0
+꙰dcF
+kER
+explain SELECT c0 FROM t0 WHERE 'C0' <= t0.c0;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t0	ALL	i0	NULL	NULL	NULL	2	Using where
+DROP INDEX i0 ON t0;
+SELECT c0 FROM t0 WHERE 'C0' <= t0.c0;
+c0
+꙰dcF
+kER
+explain SELECT c0 FROM t0 WHERE 'C0' <= t0.c0;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t0	ALL	NULL	NULL	NULL	NULL	2	Using where
+DROP TABLE t0;
+# End of 11.4 tests
diff --git a/mysql-test/main/range.test b/mysql-test/main/range.test
index dbd9d2b9cb5e2..0f9fa5549791e 100644
--- a/mysql-test/main/range.test
+++ b/mysql-test/main/range.test
@@ -2641,3 +2641,24 @@ DROP TABLE lineitem;
 --echo #
 --echo # End of 11.0 tests
 --echo #
+
+--echo #
+--echo # MDEV-39843: Prefix index causes incorrect range query result
+--echo #
+
+SET NAMES utf8mb4;
+
+CREATE TABLE t0 (c0 LONGTEXT, KEY i0 (c0(1)));
+INSERT INTO t0(c0) VALUES ('꙰dcF'), ('kER');
+
+SELECT c0 FROM t0 WHERE 'C0' <= t0.c0;
+explain SELECT c0 FROM t0 WHERE 'C0' <= t0.c0;
+
+DROP INDEX i0 ON t0;
+
+SELECT c0 FROM t0 WHERE 'C0' <= t0.c0;
+explain SELECT c0 FROM t0 WHERE 'C0' <= t0.c0;
+
+DROP TABLE t0;
+
+--echo # End of 11.4 tests
diff --git a/sql/opt_range.cc b/sql/opt_range.cc
index 0d2949adcabdc..e92ffa7adfdcb 100644
--- a/sql/opt_range.cc
+++ b/sql/opt_range.cc
@@ -9701,6 +9701,25 @@ SEL_ARG *Field::stored_field_make_mm_leaf_bounded_int(RANGE_OPT_PARAM *param,
 }
 
 
+/*
+  Check whether a partial (prefix) key segment cannot be used as a range
+  bound because of the collation.
+
+  For a collation with a complex, non one-to-one character-to-weight mapping
+  (contractions, expansions or ignorable characters: MY_CS_NON1TO1) the sort
+  key of a character prefix does not preserve the ordering of the full value,
+  so neither a lower nor an upper prefix bound is sound.
+  Equality is unaffected and keeps using the prefix (the full condition is
+  always rechecked).
+*/
+static bool prefix_key_part_unsafe_for_range(const KEY_PART *key_part,
+                                             const Field *field)
+{
+  return (key_part->flag & HA_PART_KEY_SEG) &&
+         (field->charset()->state & MY_CS_NON1TO1);
+}
+
+
 SEL_ARG *Field::stored_field_make_mm_leaf(RANGE_OPT_PARAM *param,
                                           KEY_PART *key_part,
                                           scalar_comparison_op op,
@@ -9715,12 +9734,20 @@ SEL_ARG *Field::stored_field_make_mm_leaf(RANGE_OPT_PARAM *param,
 
   switch (op) {
   case SCALAR_CMP_LE:
+    if (prefix_key_part_unsafe_for_range(key_part, this))
+      DBUG_RETURN(0);
     DBUG_RETURN(new (mem_root) SEL_ARG_LE(str, this));
   case SCALAR_CMP_LT:
+    if (prefix_key_part_unsafe_for_range(key_part, this))
+      DBUG_RETURN(0);
     DBUG_RETURN(new (mem_root) SEL_ARG_LT(thd, str, key_part, this, value));
   case SCALAR_CMP_GT:
+    if (prefix_key_part_unsafe_for_range(key_part, this))
+      DBUG_RETURN(0);
     DBUG_RETURN(new (mem_root) SEL_ARG_GT(thd, str, key_part, this, value));
   case SCALAR_CMP_GE:
+    if (prefix_key_part_unsafe_for_range(key_part, this))
+      DBUG_RETURN(0);
     DBUG_RETURN(new (mem_root) SEL_ARG_GE(thd, str, key_part, this, value));
   case SCALAR_CMP_EQ:
   case SCALAR_CMP_EQUAL: