Error verifying Prime31Editor

I'm building an iOS game and using Prime31 plugin for GameCenter integration. All is smooth on iOS but I need to submit a web demo to a third party and when I build for web this exception pops up:

 VerificationException: Error verifying Prime31Editor.MetroPostProcessBuild:onPostProcessBuildPlayer (UnityEditor.BuildTarget,string): Object not compatible with constrained type at 0x0030
 UnityEditor.HostView:OnGUI()

the build, however succeeds but the object containing an old GUI scripts (yes I'm lazy) displays nothing. now I'm absolutely sure I'm using the "#if UNITY_IPHONE" everywhere I call Prime functions. still the problem is just annoying crap out of me.

This is not crucial problem, I can find a way around it but for the sake of all that is geeky, what can be the reason of this problem? here is my GUI script:

 void OnGUI()
     {
         GUI.DrawTexture (new Rect (0, -Screen.height * 0.05f, Screen.width * 1.2f, Screen.height * 1.2f), t2DBackground);
         GUI.Box(new Rect(0, 0, Screen.width, Screen.width * (t2DHeader.width / t2DHeader.height)), t2DHeader, GUIStyle.none);
 
 
         
         //buttons
         if(GUI.Button(new Rect((Screen.width - butWidth) / 2, Screen.height * 7 / 22, butWidth, butHeight), "PLAY", mySkin.button)){
             PlayerPrefs.SetInt("GameMode", (int)GameMode.Play);
 #if UNITY_IPHONE
             XCodeBinding.AdFooterLoad (false);
 #endif
             Application.LoadLevel(1);
         }
         if(GUI.Button(new Rect((Screen.width - butWidth) / 2, Screen.height * 7 / 22 + butHeight, butWidth, butHeight), "SCORES", mySkin.button)){
 #if UNITY_IPHONE
             GameCenterBinding.showLeaderboardWithTimeScopeAndLeaderboard(GameCenterLeaderboardTimeScope.AllTime, "com.lemondo.CylinderGame.Leaderboard");
 #endif
         }
         if(GUI.Button(new Rect((Screen.width - butWidth) / 2, Screen.height * 7 / 22 + butHeight * 2, butWidth, butHeight), "REMOVE ADS", mySkin.button)){
 #if UNITY_IPHONE
             //remove ads
 #endif
         }
         if(GUI.Button(new Rect((Screen.width - butWidth) / 2, Screen.height * 7 / 22 + + butHeight * 3, butWidth, butHeight), "MORE SHIT", mySkin.button)){
 #if UNITY_IPHONE
             //more games
             XCodeBinding.GoToMores();
 #endif
         }
     }