diff --git a/scmvm/assembler.scm b/scmvm/assembler.scm
index 690531a..427fabf 100644
--- a/scmvm/assembler.scm
+++ b/scmvm/assembler.scm
@@ -2,6 +2,7 @@
   #:use-module (srfi srfi-1)
   #:use-module (scmvm vm)
   #:use-module (rnrs bytevectors)
+  #:use-module (rnrs io ports)
   #:use-module ((scheme base)
                 #:select (write-u8 write-bytevector))
   #:export (assemble))
@@ -33,17 +34,19 @@
   (bytevector-s32-native-set! bv 0 word)
   (write-bytevector bv))
 
-(define (assemble inst-seq)
-  (define labels (find-labels inst-seq 0))
-  (let loop ([seq inst-seq])
-    (cond
-     [(null? seq) '()]
-     [(label? (car seq)) (loop (cdr seq))]
-     [else
-      (let* [(inst (car seq))
-             (inst-obj (lookup-instruction (car inst)))]
-        (write-u8 (instruction-code inst-obj))
-        (case (instruction-type inst-obj)
-          [(i) (write-word (cadr inst))]
-          [(j) (write-word (assq-ref labels (cadr inst)))])
-        (loop (cdr seq)))])))
+(define (assemble inst-seq port)
+  (with-output-to-port port
+    (lambda ()
+      (define labels (find-labels inst-seq 0))
+      (let loop ([seq inst-seq])
+        (cond
+         [(null? seq) '()]
+         [(label? (car seq)) (loop (cdr seq))]
+         [else
+          (let* [(inst (car seq))
+                 (inst-obj (lookup-instruction (car inst)))]
+            (write-u8 (instruction-code inst-obj))
+            (case (instruction-type inst-obj)
+              [(i) (write-word (cadr inst))]
+              [(j) (write-word (assq-ref labels (cadr inst)))])
+            (loop (cdr seq)))])))))
diff --git a/tests.scm b/tests.scm
index 72dca79..cf922da 100644
--- a/tests.scm
+++ b/tests.scm
@@ -102,11 +102,11 @@
 (define-test-suite "assembler"
   (define-test "adder"
     (define out (open-output-bytevector))
-    (with-output-to-port out (lambda () (assemble adder-program-asm)))
+    (assemble adder-program-asm out)
     (assert-equal adder-program-bytecode (get-output-bytevector out)))
   (define-test "fib"
     (define out (open-output-bytevector))
-    (with-output-to-port out (lambda () (assemble fib-program-asm)))
+    (assemble fib-program-asm out)
     (assert-equal fib-program-bytecode (get-output-bytevector out))))
 
 (define-test-suite "vm"