diff --git a/doc/src/sgml/ref/create_table.sgml b/doc/src/sgml/ref/create_table.sgml
index 3b2b227683..8585b1abf2 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 always <literal>NOT NULL</literal>.
      </para>
 
      <para>
diff --git a/src/backend/parser/parse_utilcmd.c b/src/backend/parser/parse_utilcmd.c
index 75266caeb4..f16021a2e2 100644
--- a/src/backend/parser/parse_utilcmd.c
+++ b/src/backend/parser/parse_utilcmd.c
@@ -657,6 +657,13 @@ transformColumnDefinition(CreateStmtContext *cxt, ColumnDef *column)
 									column->colname, cxt->relation->relname),
 							 parser_errposition(cxt->pstate,
 												constraint->location)));
+				if (saw_identity)
+					ereport(ERROR,
+							(errcode(ERRCODE_SYNTAX_ERROR),
+							 errmsg("identity column \"%s\" of table \"%s\" must be NOT NULL",
+									column->colname, cxt->relation->relname),
+							 parser_errposition(cxt->pstate,
+												constraint->location)));
 				column->is_not_null = false;
 				saw_nullable = true;
 				break;
@@ -700,6 +707,14 @@ transformColumnDefinition(CreateStmtContext *cxt, ColumnDef *column)
 								(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 								 errmsg("identity columns are not supported on partitions")));
 
+					if (saw_nullable && !column->is_not_null)
+						ereport(ERROR,
+								(errcode(ERRCODE_SYNTAX_ERROR),
+								 errmsg("identity column \"%s\" of table \"%s\" must be NOT NULL",
+										column->colname, cxt->relation->relname),
+								 parser_errposition(cxt->pstate,
+													constraint->location)));
+
 					ctype = typenameType(cxt->pstate, column->typeName, NULL);
 					typeOid = ((Form_pg_type) GETSTRUCT(ctype))->oid;
 					ReleaseSysCache(ctype);
diff --git a/src/test/regress/expected/identity.out b/src/test/regress/expected/identity.out
index fbca0333a2..83e7d3a971 100644
--- a/src/test/regress/expected/identity.out
+++ b/src/test/regress/expected/identity.out
@@ -547,3 +547,14 @@ 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:  identity column "id" of table "itest15" must be NOT NULL
+LINE 1: ...ABLE itest15 (id integer GENERATED ALWAYS AS IDENTITY NULL);
+                                                                 ^
+CREATE TABLE itest15 (id integer NULL GENERATED ALWAYS AS IDENTITY); -- fail
+ERROR:  identity column "id" of table "itest15" must be NOT NULL
+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 cdda186720..48aab70c06 100644
--- a/src/test/regress/sql/identity.sql
+++ b/src/test/regress/sql/identity.sql
@@ -346,3 +346,11 @@ 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;
+