diff --git a/collects/racket/draw/private/bitmap.rkt b/collects/racket/draw/private/bitmap.rkt
index b8e046200f..d4f331d934 100644
--- a/collects/racket/draw/private/bitmap.rkt
+++ b/collects/racket/draw/private/bitmap.rkt
@@ -603,7 +603,8 @@
           (cairo_surface_flush s)
           (let ([data (cairo_image_surface_get_data s)]
                 [row-width (cairo_image_surface_get_stride s)]
-                [use-alpha? (or alpha-channel? b&w?)])
+                [use-alpha? (or alpha-channel? b&w?)]
+                [set-alpha? alpha-channel?])
             (let ([w2 (+ x (min (- width x) w))])
               (for* ([j (in-range y (min (+ y h) height))])
                 (let* ([row (* j row-width)]
@@ -612,35 +613,25 @@
                        [ri-end (+ row (* 4 w2))]
                        [pi-start p]
                        [pi-end (+ p (* 4 (- w2 x)))])
-                  (unless (and (<= 0 ri-start)
-                               (<= ri-end (bytes-length data)))
-                    (error 'bitmap.rkt "ri indicies out of range ~s & ~s vs ~s"
-                           ri-start
-                           ri-end
-                           (bytes-length data)))
-                  (unless (and (<= 0 pi-start)
-                               (<= pi-end (bytes-length bstr)))
-                    (error 'bitmap.rkt "pi indicies out of range ~s & ~s vs ~s"
-                           pi-start
-                           pi-end
-                           (bytes-length bstr)))
                   (for ([ri (in-range ri-start ri-end 4)]
                         [pi (in-range pi-start pi-end 4)])
-                    (let ([a (bytes-ref data (+ ri A))])
+                    (let ([a (unsafe-bytes-ref data (+ ri A))])
                       (let-syntax ([unmult 
-                                    (λ (stx)
-                                      (syntax-case stx ()
-                                        [(_ v)
-                                         #'(if use-alpha?
-                                               (if (unsafe-fx= 0 a)
-                                                   255
-                                                   ;; `min' shouldn't be necessary, but it's
-                                                   ;; just in case the data is ill-formed
-                                                   (unsafe-fxmin 255 (unsafe-fxquotient 
-                                                                      (unsafe-fx* v 255) 
-                                                                      a)))
-                                               v)]))])
-                      (when alpha-channel?
+                                    ;; Defined as a macro to copy the 
+                                    ;; `unsafe-bytes-ref' to each branch,
+                                    ;; instead of binding a local variable
+                                    (syntax-rules ()
+                                      [(_ v)
+                                       (if use-alpha?
+                                           (if (unsafe-fx= 0 a)
+                                               255
+                                               ;; `min' shouldn't be necessary, but it's
+                                               ;; just in case the data is ill-formed
+                                               (unsafe-fxmin 255 (unsafe-fxquotient 
+                                                                  (unsafe-fx* v 255) 
+                                                                  a)))
+                                           v)])])
+                      (when set-alpha?
                         (unsafe-bytes-set! bstr pi a))
                       (unsafe-bytes-set! bstr (+ pi 1) (unmult (unsafe-bytes-ref data (+ ri R))))
                       (unsafe-bytes-set! bstr (+ pi 2) (unmult (unsafe-bytes-ref data (+ ri G))))