diff --git a/interpreter.rkt b/interpreter.rkt
index 31155b2..b2c3d3b 100644
--- a/interpreter.rkt
+++ b/interpreter.rkt
@@ -407,19 +407,17 @@
 ; calls done when the final ProgState has been
 ; computed by executing the program
 (define (execute prog done get-input)
-  
-  ; The program length in characters
-  (define program-len (string-length (prog-state-program prog)))
 
-  ; execute-help: ProgState -> _
+  ; execute-help: ProgState ProgState -> _
   ; Helper function for `execute`.
-  ; Given an initial ProgState state, calls done
-  ; when the final ProgState is ready.
-  (define (execute-help w)
-    (cond [(>= (prog-state-ip w) program-len) (done w)]
-          [else (execute-instr w execute-help get-input)]))
+  ; Given an initial ProgState state, and the previous state,
+  ; calls done when the final ProgState is ready.
+  (define (execute-help w prev)
+    (cond [(eq? w #f) (done prev)]
+          [else (execute-instr w (lambda (new) (execute-help new w))
+                               get-input)]))
 
-  (execute-help prog))
+  (execute-instr prog (lambda (new) (execute-help new prog)) get-input))
 
 ; Tests for execute
 (test-begin