diff --git a/mysql-test/r/func_misc.result b/mysql-test/r/func_misc.result
index 39bf1470afe..946a07d0eb2 100644
--- a/mysql-test/r/func_misc.result
+++ b/mysql-test/r/func_misc.result
@@ -190,6 +190,12 @@ ERROR 21000: Operand should contain 1 column(s)
 drop table table_26093;
 drop function func_26093_a;
 drop function func_26093_b;
+create table t1 (a int not null);
+insert into t1 values (-1), (-2);
+select min(a) from t1 group by inet_ntoa(a);
+min(a)
+-2
+drop table t1;
 End of 5.0 tests
 select connection_id() > 0;
 connection_id() > 0
diff --git a/mysql-test/t/func_misc.test b/mysql-test/t/func_misc.test
index 01eff55d1f6..db4df9ef244 100644
--- a/mysql-test/t/func_misc.test
+++ b/mysql-test/t/func_misc.test
@@ -198,6 +198,14 @@ drop table table_26093;
 drop function func_26093_a;
 drop function func_26093_b;
 
+#
+# Bug #31349: ERROR 1062 (23000): Duplicate entry '' for key 'group_key'
+#
+create table t1 (a int not null);
+insert into t1 values (-1), (-2);
+select min(a) from t1 group by inet_ntoa(a);
+drop table t1;
+
 --echo End of 5.0 tests
 
 #
diff --git a/sql/item_strfunc.h b/sql/item_strfunc.h
index 155f8457156..2322f0aacc2 100644
--- a/sql/item_strfunc.h
+++ b/sql/item_strfunc.h
@@ -657,7 +657,12 @@ public:
     }
   String* val_str(String* str);
   const char *func_name() const { return "inet_ntoa"; }
-  void fix_length_and_dec() { decimals = 0; max_length=3*8+7; }
+  void fix_length_and_dec() 
+  { 
+    decimals= 0; 
+    max_length= 3 * 8 + 7; 
+    maybe_null= 1;
+  }
 };
 
 class Item_func_quote :public Item_str_func