diff --git a/stepper.py b/stepper.py
index c1b2feb..5dd008c 100644
--- a/stepper.py
+++ b/stepper.py
@@ -77,7 +77,11 @@ def step(insns, terminator, blocks, stack_frames, ssa_env, global_env, heap,
                  ' Current size is {}')
                 .format(size))
 
-        res = heap[location_v]
+        if location_v == 0:
+            err('You are not allowed to read from location 0')
+            res = 0
+        else:
+            res = heap[location_v]
     elif isinstance(next_insn, ll.Store):
         ty = next_insn.ty
         base_ty = ty2base_ty(ty, tdecs)
@@ -91,8 +95,9 @@ def step(insns, terminator, blocks, stack_frames, ssa_env, global_env, heap,
         # TODO
         print('heap[{}] <- {}'
               .format(location_v, value_v))
-
-        if size == 1:
+        if location_v == 0:
+            err('You are not allowed to store at location 0 (Null)')
+        elif size == 1:
             heap[location_v] = value_v
         else:
             err(('This emulator cannot store objects larger than 1 cell.'
@@ -407,7 +412,6 @@ define i64 @tigermain (i64 %U_mainSL_8, i64 %U_mainDummy_9) {
     terminator = first_block.terminator
     stack_frames = []
     ssa_env = {}
-    # TODO: memory structure has not been decided yet
     heap = [None]
     call_res = []