diff --git a/assemble.rkt b/assemble.rkt
index e2f8f68..dbdebeb 100644
--- a/assemble.rkt
+++ b/assemble.rkt
@@ -360,8 +360,7 @@ EOF
              (assemble-display-name (MakeCompiledProcedureShell-display-name op)))]
     
     [(ApplyPrimitiveProcedure? op)
-     (format "MACHINE.proc(MACHINE, ~a)"
-             (ApplyPrimitiveProcedure-arity op))]
+     (format "MACHINE.proc(MACHINE, MACHINE.argcount)")]
     
     [(GetControlStackLabel? op)
      (format "MACHINE.control[MACHINE.control.length-1].label")]
diff --git a/compile.rkt b/compile.rkt
index a032164..d3a58f2 100644
--- a/compile.rkt
+++ b/compile.rkt
@@ -825,7 +825,7 @@
                  (make-instruction-sequence 
                   `(,(make-PerformStatement (make-CheckPrimitiveArity! (make-Reg 'argcount)))
                     ,(make-AssignPrimOpStatement 'val 
-                                                 (make-ApplyPrimitiveProcedure number-of-arguments))
+                                                 (make-ApplyPrimitiveProcedure))
                     ,(make-PopEnvironment number-of-arguments 0)
                     ,(make-AssignImmediateStatement target (make-Reg 'val))))
                  (LabelLinkage-label after-call)))))))
diff --git a/il-structs.rkt b/il-structs.rkt
index 4628936..24c178e 100644
--- a/il-structs.rkt
+++ b/il-structs.rkt
@@ -170,9 +170,9 @@
 
 
 ;; Applies the primitive procedure that's stored in the proc register, using
-;; the arity number of values that are bound in the environment as arguments
+;; the argcount number of values that are bound in the environment as arguments
 ;; to that primitive.
-(define-struct: ApplyPrimitiveProcedure ([arity : Natural])
+(define-struct: ApplyPrimitiveProcedure ()
   #:transparent)
 
 
diff --git a/simulator.rkt b/simulator.rkt
index f21337d..264aceb 100644
--- a/simulator.rkt
+++ b/simulator.rkt
@@ -397,7 +397,7 @@
            (let: ([prim : SlotValue (machine-proc m)]
                   [args : (Listof PrimitiveValue)
                         (map ensure-primitive-value (take (machine-env m)
-                                                          (ApplyPrimitiveProcedure-arity op)))])
+                                                          (ensure-natural (machine-argcount m))))])
                  (cond
                    [(primitive-proc? prim)
                     (target-updater! m (ensure-primitive-value 
diff --git a/test-assemble.rkt b/test-assemble.rkt
index 31f3e75..79e8ad7 100644
--- a/test-assemble.rkt
+++ b/test-assemble.rkt
@@ -153,8 +153,8 @@
                                                    (make-Const 3))
                     (make-AssignImmediateStatement (make-EnvLexicalReference 1 #f)
                                                    (make-Const 4))
-                    (make-AssignPrimOpStatement 'val
-                                                (make-ApplyPrimitiveProcedure 2))
+                    (make-AssignImmediateStatement 'argcount (make-Const 2))
+                    (make-AssignPrimOpStatement 'val (make-ApplyPrimitiveProcedure))
                     'done))
       "7")
 
diff --git a/test-simulator.rkt b/test-simulator.rkt
index 354f5bf..1373558 100644
--- a/test-simulator.rkt
+++ b/test-simulator.rkt
@@ -475,7 +475,8 @@
                         ,(make-PushEnvironment 2 #f)
                         ,(make-AssignImmediateStatement (make-EnvLexicalReference 0 #f) (make-Const 126389))
                         ,(make-AssignImmediateStatement (make-EnvLexicalReference 1 #f) (make-Const 42))
-                        ,(make-AssignPrimOpStatement 'val (make-ApplyPrimitiveProcedure 2))
+                        ,(make-AssignImmediateStatement 'argcount (make-Const 2))
+                        ,(make-AssignPrimOpStatement 'val (make-ApplyPrimitiveProcedure))
                         after))])
   (test (machine-val (run m))
         (+ 126389 42))