diff --git a/mysql-test/main/spatial_utility_function_collect.result b/mysql-test/main/spatial_utility_function_collect.result
index 07095862def21..21edda95006a0 100644
--- a/mysql-test/main/spatial_utility_function_collect.result
+++ b/mysql-test/main/spatial_utility_function_collect.result
@@ -279,4 +279,22 @@ SELECT ST_ASTEXT(ST_ENVELOPE(ST_COLLECT(c))) FROM t;
 ST_ASTEXT(ST_ENVELOPE(ST_COLLECT(c)))
 NULL
 DROP TABLE t;
+#
+# MDEV-38669 ST_COLLECT reads past the end of a non-geometry argument
+#
+SELECT ST_COLLECT('');
+ST_COLLECT('')
+NULL
+SELECT ST_COLLECT('') IS NULL;
+ST_COLLECT('') IS NULL
+1
+SELECT ST_COLLECT('not wkb');
+ST_COLLECT('not wkb')
+NULL
+CREATE TABLE t1 (a VARCHAR(16));
+INSERT INTO t1 VALUES (''), ('not wkb'), (NULL);
+SELECT ST_COLLECT(a) FROM t1;
+ST_COLLECT(a)
+NULL
+DROP TABLE t1;
 # End of 12.3 tests
diff --git a/mysql-test/main/spatial_utility_function_collect.test b/mysql-test/main/spatial_utility_function_collect.test
index f9e6fbd141a70..e3bffb4a20a7e 100644
--- a/mysql-test/main/spatial_utility_function_collect.test
+++ b/mysql-test/main/spatial_utility_function_collect.test
@@ -240,4 +240,15 @@ SELECT ST_ASTEXT(ST_ENVELOPE(ST_COLLECT(c))) FROM t;
 
 DROP TABLE t;
 
+--echo #
+--echo # MDEV-38669 ST_COLLECT reads past the end of a non-geometry argument
+--echo #
+SELECT ST_COLLECT('');
+SELECT ST_COLLECT('') IS NULL;
+SELECT ST_COLLECT('not wkb');
+CREATE TABLE t1 (a VARCHAR(16));
+INSERT INTO t1 VALUES (''), ('not wkb'), (NULL);
+SELECT ST_COLLECT(a) FROM t1;
+DROP TABLE t1;
+
 --echo # End of 12.3 tests
diff --git a/sql/item_sum.cc b/sql/item_sum.cc
index efc5b9450c3f6..dc970d18c261c 100644
--- a/sql/item_sum.cc
+++ b/sql/item_sum.cc
@@ -4642,6 +4642,13 @@ bool Item_func_collect::add() {
   if (tmp_arg[0]->null_value)
     return 0;
 
+  /*
+    A value too short to hold the SRID and WKB header isn't a valid
+    geometry, so contribute nothing (NULL) to the aggregate.
+  */
+  if (!Geometry::is_valid_geometry_length(wkb->length()))
+    return 0;
+
   if(is_distinct && list_contains_element(wkb))
     return 0;
 
diff --git a/sql/spatial.cc b/sql/spatial.cc
index c8ff4d6530502..b516351b8cb6f 100644
--- a/sql/spatial.cc
+++ b/sql/spatial.cc
@@ -351,7 +351,7 @@ Geometry *Geometry::construct(Geometry_buffer *buffer,
   uint32 geom_type;
   Geometry *result;
 
-  if (data_len < SRID_SIZE + WKB_HEADER_SIZE)   // < 4 + (1 + 4)
+  if (!is_valid_geometry_length(data_len))
     return NULL;
   /* + 1 to skip the byte order (stored in position SRID_SIZE). */
   geom_type= uint4korr(data + SRID_SIZE + 1);
diff --git a/sql/spatial.h b/sql/spatial.h
index c36dce735cb59..89051b43a44be 100644
--- a/sql/spatial.h
+++ b/sql/spatial.h
@@ -325,6 +325,14 @@ class Geometry
 
   static Geometry *construct(Geometry_buffer *buffer,
                              const char *data, uint32 data_len);
+  /*
+    A WKB starts with a SRID and a WKB header.  A buffer shorter than
+    the length of these together cannot possibly hold a geometry.
+  */
+  static bool is_valid_geometry_length(uint32 data_len)
+  {
+    return data_len >= SRID_SIZE + WKB_HEADER_SIZE;
+  }
   static Geometry *create_from_wkt(Geometry_buffer *buffer,
 				   Gis_read_stream *trs, String *wkt,
 				   bool init_stream=1);