diff --git a/src/interface.jl b/src/interface.jl
index 242d367..4364522 100755
--- a/src/interface.jl
+++ b/src/interface.jl
@@ -33,7 +33,7 @@ using CommUtils
 #------------------------------------------------------------------------------------------------100
 
 @kwdef mutable struct agent
-    availableRole = ["system", "user", "assistant"]
+    availableRole::AbstractVector = ["system", "user", "assistant"]
     agentName::String = "assistant"
     maxUserMsg::Int = 10
     earlierConversation::String = ""  # summary of earlier conversation