diff --git a/src/ram.py b/src/ram.py
index 6f2ede0..97c51ba 100644
--- a/src/ram.py
+++ b/src/ram.py
@@ -42,8 +42,9 @@ class Ram(object):
             if len(self.output_registers) <= index:
                 self.output_registers.extend([None] * (index + 1 - len(self.output_registers))) # extend with uninitialized values
             self.output_registers[index] = value
+        elif type == "value":
+            raise TypeError("cannot write on value")
         elif type == "reference":
             target_index = self.read_register(ref_origin, index)
             self.write_register(ref_target, target_index, value)
 
-    # def op(self, type_r1, index_r1, type_r2, index_r2, type_r3, index_r3)