diff --git a/src/decaylanguage/data/decfile.lark b/src/decaylanguage/data/decfile.lark
index 7094b2e8..6e62023d 100644
--- a/src/decaylanguage/data/decfile.lark
+++ b/src/decaylanguage/data/decfile.lark
@@ -26,7 +26,7 @@ cdecay : "CDecay" LABEL
 
 define : "Define" LABEL SIGNED_NUMBER
 
-particle_def: "Particle" LABEL SIGNED_NUMBER SIGNED_NUMBER // Set the mass and width of a particle (in GeV)
+particle_def: "Particle" LABEL SIGNED_NUMBER SIGNED_NUMBER? // Set the mass and width (optional) of a particle (in GeV)
 
 alias : "Alias" LABEL LABEL