diff --git a/interp.py b/interp.py
index aeab99e..d3ad996 100644
--- a/interp.py
+++ b/interp.py
@@ -14,12 +14,23 @@
         return self._value
 
     def verb_Append(self, args):
+        args_count = len(args)
+        if args_count != 1:
+            raise InterpreterError("Invalid argument count %i, expected 1" % (args_count))
         appendix = args[0]
+        if appendix.__class__ != Text:
+            raise InterpreterError("Invalid argument type %s, expected Text" % (appendix.__class__.__name__))
         return Text(self.value() + " " + appendix.value())
 
 class Module_System:
     def verb_Print(self, args):
-        print(args[0].value())
+        args_count = len(args)
+        if args_count != 1:
+            raise InterpreterError("Invalid argument count %i, expected 1" % (args_count))
+        line = args[0]
+        if line.__class__ != Text:
+            raise InterpreterError("Invalid argument type %s, expected Text" % (line.__class__.__name__))
+        print(line.value())
 
     def verb_Read(self, args):
         return Text(input())