diff --git a/mysql-test/main/func_str.result b/mysql-test/main/func_str.result
index 3278cfdc835..f6769610225 100644
--- a/mysql-test/main/func_str.result
+++ b/mysql-test/main/func_str.result
@@ -5511,6 +5511,15 @@ NULL
 select hex(aes_decrypt(x'9E6F76516B4DE68FED7A77632FC0913D', 'bar', 'abcdefghabcdefgh', 'aes-128-ctr')) `expected garbage`;
 expected garbage
 98D7BC3151620F384B0A953686AF37C9
+set @@block_encryption_mode=default;
+#
+# MDEV-31633 Assertion `!item->null_value' failed in Type_handler::Item_send_str
+#
+select aes_encrypt(a,a) is null from (values('a'),(NULL),('b')) x;
+aes_encrypt(a,a) is null
+0
+1
+0
 #
 # End of 11.2 tests
 #
diff --git a/mysql-test/main/func_str.test b/mysql-test/main/func_str.test
index de5b871729c..feb3057ec41 100644
--- a/mysql-test/main/func_str.test
+++ b/mysql-test/main/func_str.test
@@ -2456,6 +2456,13 @@ select aes_decrypt(x'9E6F76516B4DE68FED7A77632FC0913D', 'bar', 'abcdefghabcdefgh
 # but ctr doesn't use padding, so:
 select hex(aes_decrypt(x'9E6F76516B4DE68FED7A77632FC0913D', 'bar', 'abcdefghabcdefgh', 'aes-128-ctr')) `expected garbage`;
 
+set @@block_encryption_mode=default;
+
+--echo #
+--echo # MDEV-31633 Assertion `!item->null_value' failed in Type_handler::Item_send_str
+--echo #
+select aes_encrypt(a,a) is null from (values('a'),(NULL),('b')) x;
+
 --echo #
 --echo # End of 11.2 tests
 --echo #
diff --git a/sql/item_strfunc.cc b/sql/item_strfunc.cc
index cb3138d4471..34fc768846e 100644
--- a/sql/item_strfunc.cc
+++ b/sql/item_strfunc.cc
@@ -405,6 +405,7 @@ String *Item_aes_crypt::val_str(String *str2)
         str2->length((uint) aes_length);
         DBUG_ASSERT(collation.collation == &my_charset_bin);
         str2->set_charset(&my_charset_bin);
+        null_value= 0;
         return str2;
       }
     }