diff --git a/mysql-test/r/func_group.result b/mysql-test/r/func_group.result
index 3a2cb26910a..e7f27ebb07e 100644
--- a/mysql-test/r/func_group.result
+++ b/mysql-test/r/func_group.result
@@ -1387,4 +1387,9 @@ SELECT 1 FROM t1 GROUP BY (SELECT SLEEP(0) FROM t1 ORDER BY AVG(DISTINCT a) );
 1
 1
 DROP TABLE t1;
+CREATE TABLE t1 (a int, b date NOT NULL, KEY k1 (a,b));
+SELECT MIN(b) FROM t1 WHERE a=1 AND b>'2007-08-01';
+MIN(b)
+NULL
+DROP TABLE t1;
 End of 5.0 tests
diff --git a/mysql-test/t/func_group.test b/mysql-test/t/func_group.test
index 8c020eb3dc8..7e115707625 100644
--- a/mysql-test/t/func_group.test
+++ b/mysql-test/t/func_group.test
@@ -873,5 +873,14 @@ SELECT 1 FROM t1 GROUP BY (SELECT SLEEP(0) FROM t1 ORDER BY AVG(DISTINCT a) );
 
 DROP TABLE t1;
 
+#
+# Bug #30715: Assertion failed: item_field->field->real_maybe_null(), file
+# .\opt_sum.cc, line
+#
+
+CREATE TABLE t1 (a int, b date NOT NULL, KEY k1 (a,b));
+SELECT MIN(b) FROM t1 WHERE a=1 AND b>'2007-08-01';
+DROP TABLE t1;
+
 ###
 --echo End of 5.0 tests
diff --git a/sql/opt_sum.cc b/sql/opt_sum.cc
index b9de54dbf5c..3fc62d05ae5 100644
--- a/sql/opt_sum.cc
+++ b/sql/opt_sum.cc
@@ -249,20 +249,20 @@ int opt_sum_query(TABLE_LIST *tables, List<Item> &all_fields,COND *conds)
                  Check if case 1 from above holds. If it does, we should read
                  the skipped tuple.
               */
-              if (ref.key_buff[prefix_len] == 1 && 
-                  /* 
+              if (item_field->field->real_maybe_null() &&
+                  ref.key_buff[prefix_len] == 1 &&
+                  /*
                      Last keypart (i.e. the argument to MIN) is set to NULL by
                      find_key_for_maxmin only if all other keyparts are bound
                      to constants in a conjunction of equalities. Hence, we
                      can detect this by checking only if the last keypart is
                      NULL.
-                  */                     
+                  */
                   (error == HA_ERR_KEY_NOT_FOUND ||
                    key_cmp_if_same(table, ref.key_buff, ref.key, prefix_len)))
               {
-                DBUG_ASSERT(item_field->field->real_maybe_null());
                 error= table->file->index_read(table->record[0], ref.key_buff,
-                                               ref.key_length, 
+                                               ref.key_length,
                                                HA_READ_KEY_EXACT);
               }
             }