diff --git a/src/Attr.cc b/src/Attr.cc
index 6e80a923a1..e854b635e0 100644
--- a/src/Attr.cc
+++ b/src/Attr.cc
@@ -22,6 +22,7 @@ const char* attr_name(AttrTag t)
 		"&group", "&log", "&error_handler", "&type_column",
 		"(&tracked)", "&on_change", "&broker_store",
 		"&broker_allow_complex_type", "&backend", "&deprecated",
+		"&is_set",
 	};
 
 	return attr_names[int(t)];
@@ -333,6 +334,7 @@ void Attributes::CheckAttr(Attr* a)
 	switch ( a->Tag() ) {
 	case ATTR_DEPRECATED:
 	case ATTR_REDEF:
+	case ATTR_IS_SET:
 		break;
 
 	case ATTR_OPTIONAL:
diff --git a/src/Attr.h b/src/Attr.h
index ab0197057b..f883abe91c 100644
--- a/src/Attr.h
+++ b/src/Attr.h
@@ -46,6 +46,7 @@ enum AttrTag {
 	ATTR_BROKER_STORE_ALLOW_COMPLEX, // for Broker store backed tables
 	ATTR_BACKEND, // for Broker store backed tables
 	ATTR_DEPRECATED,
+	ATTR_IS_SET, // to suppress warnings
 	NUM_ATTRS // this item should always be last
 };
 
diff --git a/src/parse.y b/src/parse.y
index 8b1bcbfbfe..6429d9250f 100644
--- a/src/parse.y
+++ b/src/parse.y
@@ -5,7 +5,7 @@
 // Switching parser table type fixes ambiguity problems.
 %define lr.type ielr
 
-%expect 129
+%expect 135
 
 %token TOK_ADD TOK_ADD_TO TOK_ADDR TOK_ANY
 %token TOK_ATENDIF TOK_ATELSE TOK_ATIF TOK_ATIFDEF TOK_ATIFNDEF
@@ -28,6 +28,7 @@
 %token TOK_ATTR_BROKER_STORE_ALLOW_COMPLEX TOK_ATTR_BACKEND
 %token TOK_ATTR_PRIORITY TOK_ATTR_LOG TOK_ATTR_ERROR_HANDLER
 %token TOK_ATTR_TYPE_COLUMN TOK_ATTR_DEPRECATED
+%token TOK_ATTR_IS_SET
 
 %token TOK_DEBUG
 
@@ -1471,6 +1472,8 @@ attr:
 			{ $$ = new zeek::detail::Attr(zeek::detail::ATTR_OPTIONAL); }
 	|	TOK_ATTR_REDEF
 			{ $$ = new zeek::detail::Attr(zeek::detail::ATTR_REDEF); }
+	|	TOK_ATTR_IS_SET
+			{ $$ = new zeek::detail::Attr(zeek::detail::ATTR_IS_SET); }
 	|	TOK_ATTR_ADD_FUNC '=' expr
 			{ $$ = new zeek::detail::Attr(zeek::detail::ATTR_ADD_FUNC, {zeek::AdoptRef{}, $3}); }
 	|	TOK_ATTR_DEL_FUNC '=' expr
diff --git a/src/scan.l b/src/scan.l
index 71e95240e9..15777124d7 100644
--- a/src/scan.l
+++ b/src/scan.l
@@ -280,11 +280,12 @@ when	return TOK_WHEN;
 &default	return TOK_ATTR_DEFAULT;
 &delete_func	return TOK_ATTR_DEL_FUNC;
 &deprecated	return TOK_ATTR_DEPRECATED;
-&raw_output return TOK_ATTR_RAW_OUTPUT;
+&raw_output	return TOK_ATTR_RAW_OUTPUT;
 &error_handler	return TOK_ATTR_ERROR_HANDLER;
 &expire_func	return TOK_ATTR_EXPIRE_FUNC;
 &log		return TOK_ATTR_LOG;
 &optional	return TOK_ATTR_OPTIONAL;
+&is_set		return TOK_ATTR_IS_SET;
 &priority	return TOK_ATTR_PRIORITY;
 &type_column	return TOK_ATTR_TYPE_COLUMN;
 &read_expire	return TOK_ATTR_EXPIRE_READ;