diff --git a/mysql-test/r/subselect_sj.result b/mysql-test/r/subselect_sj.result
index b1b0bde7c57..f5c3f84bcd7 100644
--- a/mysql-test/r/subselect_sj.result
+++ b/mysql-test/r/subselect_sj.result
@@ -2152,4 +2152,21 @@ c	c
 set optimizer_prune_level= @opl_901399;
 set optimizer_switch= @os_091399;
 DROP TABLE t1,t2;
+#
+# BUG#912510: Crash in do_copy_not_null with semijoin=ON, firstmatch=ON, aggregate ...
+#
+CREATE TABLE t1 ( a VARCHAR(1) NOT NULL );
+INSERT INTO t1 VALUES ('k'),('l');
+CREATE TABLE t2 ( b VARCHAR(1) NOT NULL, KEY(b) );
+INSERT INTO t2 VALUES ('k'),('l');
+CREATE TABLE t3 ( c VARCHAR(1) NOT NULL, KEY(c) );
+INSERT INTO t3 VALUES ('m'),('n');
+SELECT a, COUNT(*) FROM t1
+WHERE a IN (
+SELECT b FROM t2 force index(b), t3 force index(c)
+WHERE c = b AND b = a
+);
+a	COUNT(*)
+NULL	0
+DROP TABLE t1, t2, t3;
 set optimizer_switch=@subselect_sj_tmp;
diff --git a/mysql-test/r/subselect_sj_jcl6.result b/mysql-test/r/subselect_sj_jcl6.result
index 2df7d99ee64..21a8ea39457 100644
--- a/mysql-test/r/subselect_sj_jcl6.result
+++ b/mysql-test/r/subselect_sj_jcl6.result
@@ -2166,6 +2166,23 @@ c	c
 set optimizer_prune_level= @opl_901399;
 set optimizer_switch= @os_091399;
 DROP TABLE t1,t2;
+#
+# BUG#912510: Crash in do_copy_not_null with semijoin=ON, firstmatch=ON, aggregate ...
+#
+CREATE TABLE t1 ( a VARCHAR(1) NOT NULL );
+INSERT INTO t1 VALUES ('k'),('l');
+CREATE TABLE t2 ( b VARCHAR(1) NOT NULL, KEY(b) );
+INSERT INTO t2 VALUES ('k'),('l');
+CREATE TABLE t3 ( c VARCHAR(1) NOT NULL, KEY(c) );
+INSERT INTO t3 VALUES ('m'),('n');
+SELECT a, COUNT(*) FROM t1
+WHERE a IN (
+SELECT b FROM t2 force index(b), t3 force index(c)
+WHERE c = b AND b = a
+);
+a	COUNT(*)
+NULL	0
+DROP TABLE t1, t2, t3;
 set optimizer_switch=@subselect_sj_tmp;
 #
 # BUG#49129: Wrong result with IN-subquery with join_cache_level=6 and firstmatch=off
diff --git a/mysql-test/t/subselect_sj.test b/mysql-test/t/subselect_sj.test
index aee61e9bca3..f34cf5ba338 100644
--- a/mysql-test/t/subselect_sj.test
+++ b/mysql-test/t/subselect_sj.test
@@ -1997,6 +1997,25 @@ set optimizer_switch= @os_091399;
 
 DROP TABLE t1,t2;
 
+--echo #
+--echo # BUG#912510: Crash in do_copy_not_null with semijoin=ON, firstmatch=ON, aggregate ...
+--echo #
+CREATE TABLE t1 ( a VARCHAR(1) NOT NULL );
+INSERT INTO t1 VALUES ('k'),('l');
+
+CREATE TABLE t2 ( b VARCHAR(1) NOT NULL, KEY(b) );
+INSERT INTO t2 VALUES ('k'),('l');
+
+CREATE TABLE t3 ( c VARCHAR(1) NOT NULL, KEY(c) );
+INSERT INTO t3 VALUES ('m'),('n');
+
+SELECT a, COUNT(*) FROM t1
+  WHERE a IN (
+    SELECT b FROM t2 force index(b), t3 force index(c)
+    WHERE c = b AND b = a
+  );
+
+DROP TABLE t1, t2, t3;
 
 # The following command must be the last one the file 
 set optimizer_switch=@subselect_sj_tmp;
diff --git a/sql/field_conv.cc b/sql/field_conv.cc
index ce914f7b368..ed910c4ef70 100644
--- a/sql/field_conv.cc
+++ b/sql/field_conv.cc
@@ -248,6 +248,25 @@ static void do_outer_field_null(Copy_field *copy)
   }
 }
 
+/*
+  Copy: (not-NULL field in table that can be NULL-complemented) -> (not-NULL
+  field)
+*/
+static void do_copy_nullable_row_to_notnull(Copy_field *copy)
+{
+  if (*copy->null_row ||
+      (copy->from_null_ptr && (*copy->from_null_ptr & copy->from_bit)))
+  {
+    copy->to_field->set_warning(MYSQL_ERROR::WARN_LEVEL_WARN,
+                                WARN_DATA_TRUNCATED, 1);
+    copy->to_field->reset();
+  }
+  else
+  {
+    (copy->do_copy2)(copy);
+  }
+
+}
 
 /* Copy: (NULL-able field) -> (not NULL-able field) */
 static void do_copy_not_null(Copy_field *copy)
@@ -638,7 +657,15 @@ void Copy_field::set(Field *to,Field *from,bool save)
       else if (to_field == to_field->table->next_number_field)
         do_copy= do_copy_next_number;
       else
-        do_copy= do_copy_not_null;
+      {
+        if (!from_null_ptr)
+        {
+          null_row= &from->table->null_row;
+          do_copy= do_copy_nullable_row_to_notnull;
+        }
+        else
+          do_copy= do_copy_not_null;
+      }
     }
   }
   else if (to_field->real_maybe_null())
diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index cb620495897..c332b13dff6 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -646,6 +646,9 @@ JOIN::prepare(Item ***rref_pointer_array,
       aggregate functions and non-aggregate fields, any non-aggregated field
       may produce a NULL value. Set all fields of each table as nullable before
       semantic analysis to take into account this change of nullability.
+
+      Note: this loop doesn't touch tables inside merged semi-joins, because
+      subquery-to-semijoin conversion has not been done yet. This is intended.
     */
     if (mixed_implicit_grouping)
       tbl->table->maybe_null= 1;