diff --git a/libcorpus2/io/writer.h b/libcorpus2/io/writer.h
index 2c508beb6f0fcbf8507ee5bd2560a5bd4f51e959..ae9b2cd0b0067cc9ecd00da5008c580b8bdc2a0e 100644
--- a/libcorpus2/io/writer.h
+++ b/libcorpus2/io/writer.h
@@ -54,7 +54,7 @@ public:
 		return os_;
 	}
 
-	const Tagset& tagset() {
+	const Tagset& tagset() const {
 		return tagset_;
 	}