diff --git a/src/common/backend/parser/gram.y b/src/common/backend/parser/gram.y
index 7862cc7bf..f0b857684 100644
--- a/src/common/backend/parser/gram.y
+++ b/src/common/backend/parser/gram.y
@@ -6165,6 +6165,9 @@ copy_generic_opt_list:
 copy_generic_opt_elem:
 			ColLabel copy_generic_opt_arg
 				{
+					/* Character "when_expr" may be injected as "COPY ... WHEN ... "*/
+					if (pg_strcasecmp($1, "when_expr") == 0)
+						ereport(ERROR, (errcode(ERRCODE_SYNTAX_ERROR), errmsg("option \"%s\" not recognized", $1)));
 					$$ = makeDefElem($1, $2);
 				}
 		;
diff --git a/src/test/regress/expected/copyselect.out b/src/test/regress/expected/copyselect.out
index 1aaa75897..1fdfa3b56 100644
--- a/src/test/regress/expected/copyselect.out
+++ b/src/test/regress/expected/copyselect.out
@@ -60,6 +60,13 @@ ERROR:  syntax error at or near "("
 LINE 1: copy (select * from test1) (t,id) to stdout;
                                    ^
 --
+-- Test when_expr
+--
+copy (select * from test1) to stdout with (when_expr); -- error
+ERROR:  option "when_expr" not recognized
+copy (select * from test1) to stdout with (when_expr''); -- error
+ERROR:  option "when_expr" not recognized
+--
 -- Test JOIN
 --
 copy (select * from test1 join test2 using (id) order by 1) to stdout;
diff --git a/src/test/regress/sql/copyselect.sql b/src/test/regress/sql/copyselect.sql
index 92d64c5c6..34cfb8265 100644
--- a/src/test/regress/sql/copyselect.sql
+++ b/src/test/regress/sql/copyselect.sql
@@ -47,6 +47,11 @@ copy (select * from test1) from stdin;
 --
 copy (select * from test1) (t,id) to stdout;
 --
+-- Test when_expr
+--
+copy (select * from test1) to stdout with (when_expr); -- error
+copy (select * from test1) to stdout with (when_expr''); -- error
+--
 -- Test JOIN
 --
 copy (select * from test1 join test2 using (id) order by 1) to stdout;