diff --git a/interpreter/interp.rkt b/interpreter/interp.rkt
index 466c4b0..02462fb 100644
--- a/interpreter/interp.rkt
+++ b/interpreter/interp.rkt
@@ -84,19 +84,23 @@
           ;; call the external function
           (let* ([func (hash-ref runtime external-function)]
                  [sp (hash-ref registers 'rsp)]
-                 [result (func registers memory sp)]
-                 [_ (unless (integer? result)
-                      (raise-user-error 'step
-                                        "result of external function '~a' not an integer; got ~v"
-                                        external-function
-                                        result))]
-                 [_ (unless (<= (integer-length result) (word-size-bits))
-                      (raise-user-error 'step
-                                        "integer result of external function '~a' too large; got ~v"
-                                        external-function
-                                        result))]
-                 [new-registers (hash-set registers 'rax result)])
-            (make-state #:with-registers new-registers))]
+                 [result (func registers memory sp)])
+            (cond
+              [(void? result)
+               (make-state)]
+              [(not (integer? result))
+               (raise-user-error 'step
+                                 "result of external function '~a' not #<void> or an integer; got ~v"
+                                 external-function
+                                 result)]
+              [(not (<= (integer-length result) (word-size-bits)))
+               (raise-user-error 'step
+                                 "integer result of external function '~a' too large; got ~v"
+                                 external-function
+                                 result)]
+              [else
+               (let ([new-registers (hash-set registers 'rax result)])
+                 (make-state #:with-registers new-registers))]))]
          [(Call dst)
           ;; Mem.push(ip + wordsize)
           ;; ip = dst