diff --git a/src/lib/index.ts b/src/lib/index.ts
index 515d26f..0481d60 100644
--- a/src/lib/index.ts
+++ b/src/lib/index.ts
@@ -564,6 +564,12 @@ export function format(
 
 		if (node.has_prelude && is_selector_list(node.prelude)) {
 			let list = print_rule_selectors(node.prelude)
+			// An empty selector list (malformed/unknown block) prints no selector
+			// lines, so the indentation that print_rule_selectors would normally add
+			// per-selector has to be added here instead.
+			if (list === EMPTY_STRING) {
+				list = indent(depth)
+			}
 
 			let comment = get_comment(node.first_child?.end, node.block?.start)
 			if (comment) {
diff --git a/test/rules.test.ts b/test/rules.test.ts
index 32ba894..6500b28 100644
--- a/test/rules.test.ts
+++ b/test/rules.test.ts
@@ -176,7 +176,7 @@ test('formats unknown stuff in curly braces', () => {
 		}
 	`)
 	let expected = `selector {
- {
+	 {
 		color: red;
 	}
 }`