diff --git a/guix-data-service/web/controller.scm b/guix-data-service/web/controller.scm
index ceef532..34a7893 100644
--- a/guix-data-service/web/controller.scm
+++ b/guix-data-service/web/controller.scm
@@ -278,8 +278,9 @@
         (list (build-response
                #:code 200
                #:headers '((content-type . (text/plain))))
-              (lambda (port)
-                (write-metrics registry port)))))))
+              (call-with-output-string
+                (lambda (port)
+                  (write-metrics registry port))))))))
 
 (define (render-derivation derivation-file-name)
   (letpar& ((derivation
diff --git a/guix-data-service/web/nar/controller.scm b/guix-data-service/web/nar/controller.scm
index ba8b890..2164860 100644
--- a/guix-data-service/web/nar/controller.scm
+++ b/guix-data-service/web/nar/controller.scm
@@ -112,17 +112,18 @@
                      #:code 200
                      #:headers '((content-type . (application/x-nix-archive
                                                   (charset . "ISO-8859-1")))))
-                    (lambda (port)
-                      (write-file-tree
-                       file-name
-                       port
-                       #:file-type+size
-                       (lambda (file)
-                         (values 'regular
-                                 (bytevector-length derivation-bytevector)))
-                       #:file-port
-                       (lambda (file)
-                         (open-bytevector-input-port derivation-bytevector))))))))
+                    (call-with-output-bytevector
+                     (lambda (port)
+                       (write-file-tree
+                        file-name
+                        port
+                        #:file-type+size
+                        (lambda (file)
+                          (values 'regular
+                                  (bytevector-length derivation-bytevector)))
+                        #:file-port
+                        (lambda (file)
+                          (open-bytevector-input-port derivation-bytevector)))))))))
    (not-found (request-uri request))))
 
 (define (render-lzip-nar request
@@ -137,9 +138,12 @@
           (lambda (data)
             (list (build-response
                    #:code 200
-                   #:headers '((content-type . (application/x-nix-archive
-                                                (charset . "ISO-8859-1")))))
+                   #:headers `((content-type . (application/x-nix-archive
+                                                (charset . "ISO-8859-1")))
+                               (content-length . ,(bytevector-length data))))
                   (lambda (port)
+                    ;; TODO: This should probably be handled by guile-fibers
+                    (set-port-encoding! port "ISO-8859-1")
                     (put-bytevector port data)))))
    (not-found (request-uri request))))
 
@@ -188,11 +192,9 @@
                                    (lambda (file)
                                      (open-bytevector-input-port derivation-bytevector)))
                                   (get-bytevector)))))
-                        (lambda (port)
-                          (display (narinfo-string derivation-file-name
-                                                   nar-bytevector
-                                                   derivation-references)
-                                   port))))))))
+                        (narinfo-string derivation-file-name
+                                        nar-bytevector
+                                        derivation-references)))))))
    (and=> (parallel-via-thread-pool-channel
            (with-thread-postgresql-connection
             (lambda (conn)
@@ -204,14 +206,12 @@
              (list (build-response
                     #:code 200
                     #:headers '((content-type . (application/x-narinfo))))
-                   (lambda (port)
-                     (display (derivation-source-file-narinfo-string store-path
-                                                                     compression
-                                                                     compressed-size
-                                                                     hash-algorithm
-                                                                     hash
-                                                                     uncompressed-size)
-                              port))))))
+                   (derivation-source-file-narinfo-string store-path
+                                                          compression
+                                                          compressed-size
+                                                          hash-algorithm
+                                                          hash
+                                                          uncompressed-size)))))
    (not-found (request-uri request))))