diff --git a/frappy_psi/mercury.py b/frappy_psi/mercury.py
index bd958a82..86ee1c0e 100644
--- a/frappy_psi/mercury.py
+++ b/frappy_psi/mercury.py
@@ -66,7 +66,7 @@ class IO(StringIO):
     timeout = 5
     encoding = 'latin1'
 
-    @Command(StringType(), result=StringType(isUTF8=True))
+    @Command(StringType(isUTF8=True), result=StringType(isUTF8=True))
     def communicate(self, cmd, noreply=False):
         return super().communicate(cmd, noreply)