diff --git a/SlackAPI/SlackClient.cs b/SlackAPI/SlackClient.cs
index 8245096..546ae67 100644
--- a/SlackAPI/SlackClient.cs
+++ b/SlackAPI/SlackClient.cs
@@ -597,8 +597,8 @@ public void PostMessage(
             bool unfurl_links = false,
             string icon_url = null,
             string icon_emoji = null,
-            bool as_user = false,
-	    string thread_ts = null)
+            bool? as_user = null,
+	          string thread_ts = null)
         {
             List<Tuple<string,string>> parameters = new List<Tuple<string,string>>();
 
@@ -631,7 +631,8 @@ public void PostMessage(
             if (!string.IsNullOrEmpty(icon_emoji))
                 parameters.Add(new Tuple<string, string>("icon_emoji", icon_emoji));
 
-            parameters.Add(new Tuple<string, string>("as_user", as_user.ToString()));
+            if (as_user.HasValue)
+                parameters.Add(new Tuple<string, string>("as_user", as_user.ToString()));
 
             if (!string.IsNullOrEmpty(thread_ts))
                 parameters.Add(new Tuple<string, string>("thread_ts", thread_ts));