diff --git a/mtl/parser.py b/mtl/parser.py
index 29f0c97..c64714f 100644
--- a/mtl/parser.py
+++ b/mtl/parser.py
@@ -44,7 +44,7 @@ interval = "[" __ const_or_unbound __ "," __ const_or_unbound __ "]"
 
 const_or_unbound = const / "inf" / id
 
-AP = ~r"[a-z][a-z\\d]*"
+AP = ~r"[a-z_][a-z_\\d]*"
 
 bot = "FALSE"
 top = "TRUE"