diff --git a/testdata/parser/negative/25.pdl b/testdata/parser/negative/25.pdl
new file mode 100644
index 0000000000000000000000000000000000000000..621eb43c219cf9628496e49592b7f9a3ca8178e6
--- /dev/null
+++ b/testdata/parser/negative/25.pdl
@@ -0,0 +1,11 @@
+#version 100
+
+primitive main(in a, out b) {
+	int x = 0;
+	int y = 0;
+	while (true) {
+		synchronous (int x) { // illegal
+			assert fires(a) == fires(b);
+		}
+	}
+}
\ No newline at end of file