diff --git a/src/dat/nbt.h b/src/dat/nbt.h
index 805dd8b..a013108 100644
--- a/src/dat/nbt.h
+++ b/src/dat/nbt.h
@@ -44,7 +44,7 @@ struct nbt_path {
 atrb_const int nbt_isprim(u8 tag);
 
 /* gets the byte size of an NBT tag's data (excluding id and name), returns `0` upon error. */
-atrb_const size_t nbt_tagdatlen(const u8 *buf);
+atrb_pure size_t nbt_tagdatlen(const u8 *buf);
 
 /* gets the tag size of primitive types, returns `>0` on success, `<0` on failure */
 atrb_const int nbt_primsize(u8 tag);