diff --git a/doc/src/sgml/ref/create_table.sgml b/doc/src/sgml/ref/create_table.sgml
index 3b2b227683a..71703da85ae 100644
--- a/doc/src/sgml/ref/create_table.sgml
+++ b/doc/src/sgml/ref/create_table.sgml
@@ -840,6 +840,7 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       column</firstterm>.  It will have an implicit sequence attached to it
       and the column in new rows will automatically have values from the
       sequence assigned to it.
+      Such a column is implicitly <literal>NOT NULL</literal>.
      </para>
 
      <para>
diff --git a/src/backend/parser/parse_utilcmd.c b/src/backend/parser/parse_utilcmd.c
index 75266caeb4b..d56f81c79ff 100644
--- a/src/backend/parser/parse_utilcmd.c
+++ b/src/backend/parser/parse_utilcmd.c
@@ -719,7 +719,17 @@ transformColumnDefinition(CreateStmtContext *cxt, ColumnDef *column)
 
 					column->identity = constraint->generated_when;
 					saw_identity = true;
+
+					/* An identity column is implicitly NOT NULL */
+					if (saw_nullable && !column->is_not_null)
+						ereport(ERROR,
+								(errcode(ERRCODE_SYNTAX_ERROR),
+								 errmsg("conflicting NULL/NOT NULL declarations for column \"%s\" of table \"%s\"",
+										column->colname, cxt->relation->relname),
+								 parser_errposition(cxt->pstate,
+													constraint->location)));
 					column->is_not_null = true;
+					saw_nullable = true;
 					break;
 				}
 
diff --git a/src/test/regress/expected/identity.out b/src/test/regress/expected/identity.out
index fbca0333a2c..99811570b7b 100644
--- a/src/test/regress/expected/identity.out
+++ b/src/test/regress/expected/identity.out
@@ -547,3 +547,16 @@ CREATE TABLE itest14 (id serial);
 ALTER TABLE itest14 ALTER id DROP DEFAULT;
 ALTER TABLE itest14 ALTER id ADD GENERATED BY DEFAULT AS IDENTITY;
 INSERT INTO itest14 (id) VALUES (DEFAULT);
+-- Identity columns must be NOT NULL (cf bug #16913)
+CREATE TABLE itest15 (id integer GENERATED ALWAYS AS IDENTITY NULL); -- fail
+ERROR:  conflicting NULL/NOT NULL declarations for column "id" of table "itest15"
+LINE 1: ...ABLE itest15 (id integer GENERATED ALWAYS AS IDENTITY NULL);
+                                                                 ^
+CREATE TABLE itest15 (id integer NULL GENERATED ALWAYS AS IDENTITY); -- fail
+ERROR:  conflicting NULL/NOT NULL declarations for column "id" of table "itest15"
+LINE 1: CREATE TABLE itest15 (id integer NULL GENERATED ALWAYS AS ID...
+                                              ^
+CREATE TABLE itest15 (id integer GENERATED ALWAYS AS IDENTITY NOT NULL);
+DROP TABLE itest15;
+CREATE TABLE itest15 (id integer NOT NULL GENERATED ALWAYS AS IDENTITY);
+DROP TABLE itest15;
diff --git a/src/test/regress/sql/identity.sql b/src/test/regress/sql/identity.sql
index cdda186720d..52800f265c2 100644
--- a/src/test/regress/sql/identity.sql
+++ b/src/test/regress/sql/identity.sql
@@ -346,3 +346,12 @@ CREATE TABLE itest14 (id serial);
 ALTER TABLE itest14 ALTER id DROP DEFAULT;
 ALTER TABLE itest14 ALTER id ADD GENERATED BY DEFAULT AS IDENTITY;
 INSERT INTO itest14 (id) VALUES (DEFAULT);
+
+-- Identity columns must be NOT NULL (cf bug #16913)
+
+CREATE TABLE itest15 (id integer GENERATED ALWAYS AS IDENTITY NULL); -- fail
+CREATE TABLE itest15 (id integer NULL GENERATED ALWAYS AS IDENTITY); -- fail
+CREATE TABLE itest15 (id integer GENERATED ALWAYS AS IDENTITY NOT NULL);
+DROP TABLE itest15;
+CREATE TABLE itest15 (id integer NOT NULL GENERATED ALWAYS AS IDENTITY);
+DROP TABLE itest15;