diff --git a/script/RobotTCP.py b/script/RobotTCP.py
index 8f1cd89..6a6dcb4 100644
--- a/script/RobotTCP.py
+++ b/script/RobotTCP.py
@@ -54,7 +54,8 @@ class RobotTCP(TcpDevice, Stoppable):
          
     def evaluate(self, cmd, timeout=None):
         ret = self.execute('eval', cmd, timeout=timeout)
-        if ret.strip() != "": raise Exception(ret)
+        if type(ret) is str:
+            if ret.strip() != "": raise Exception(ret)
 
     def get_var(self, name):
         return self.execute('get_var', name)