diff --git a/pollen/private/preheat-cache.rkt b/pollen/private/preheat-cache.rkt
index 08e86a7..e486d60 100644
--- a/pollen/private/preheat-cache.rkt
+++ b/pollen/private/preheat-cache.rkt
@@ -1,55 +1,62 @@
 #lang racket/base
-(require "file-utils.rkt" racket/file "cache-utils.rkt" "debug.rkt" racket/path racket/place sugar/list)
+(require racket/file
+         racket/path
+         racket/place
+         racket/list
+         sugar/list
+         "file-utils.rkt"
+         "cache-utils.rkt"
+         "debug.rkt")
 (provide preheat-cache)
 
+(define (path-cached? path)
+  ;; #true = already cached; #false = not cached
+  ;; seems like it would be slow to load cache.rktd but it's not.
+  (define-values (_ private-cache-dir) (make-cache-dirs path))
+  (define cache-db-file (build-path private-cache-dir "cache.rktd"))
+  (or (file-exists? cache-db-file)
+      (hash-has-key? (file->value cache-db-file) (paths->key path))))
+
+;; compile a path inside a place (= parallel processing)
+(define (path-into-place starting-dir path)
+  (message (format "caching: ~a" (find-relative-path starting-dir path)))
+  (define p
+    (place ch
+           (define path (place-channel-get ch))
+           (define-values (_ path-name __) (split-path path))
+           (message (format "compiling: ~a" path))
+           ;; use #false to signal compile error. Otherwise allow errors to pass.
+           (define result
+             (with-handlers ([exn:fail? (λ (e) (message (format "compile failed: ~a" path-name)) #false)])
+               (path->hash path)))
+           (place-channel-put ch result)))
+  (place-channel-put p path)
+  p)
+
 (define (preheat-cache starting-dir)
   (unless (and (path-string? starting-dir) (directory-exists? starting-dir))
-    (error 'preheat-cache (format "~a is not a directory" starting-dir)))
-  
+    (raise-argument-error 'preheat-cache "directory" starting-dir))
   (define max-places (processor-count)) ; number of parallel processes to spawn at a time
-  
-  (define paths-that-should-be-cached (for/list ([path (in-directory starting-dir)]
-                                                 #:when (for/or ([proc (in-list (list preproc-source?
-                                                                                      markup-source?
-                                                                                      markdown-source?
-                                                                                      pagetree-source?))])
-                                                                (proc path)))
-                                                path))
-  
+  (define paths-that-should-be-cached
+    (for/list ([path (in-directory starting-dir)]
+               #:when (for/or ([proc (in-list (list preproc-source?
+                                                    markup-source?
+                                                    markdown-source?
+                                                    pagetree-source?))])
+                              (proc path)))
+              path))
+
   ;; if a file is already in the cache, no need to hit it again.
   ;; this allows partially completed preheat jobs to resume.
-  (define uncached-paths (filter
-                          (λ (path)
-                            ;; #t = not cached; #f = already cached
-                            ;; seems like it would be slow to load cache.rktd but it's not.
-                            (define-values (_ private-cache-dir) (make-cache-dirs path))
-                            (define cache-db-file (build-path private-cache-dir "cache.rktd"))
-                            (cond
-                              [(not (file-exists? cache-db-file)) #t]
-                              [else (define cache-db (file->value cache-db-file))
-                                    (not (hash-has-key? cache-db (paths->key path)))])) paths-that-should-be-cached))
-  
-  ;; compile a path inside a place (= parallel processing)
-  (define (path-into-place path)
-    (message (format "caching: ~a" (find-relative-path starting-dir path)))
-    (define p (place ch
-                     (define path (place-channel-get ch))
-                     (define-values (path-dir path-name _) (split-path path))
-                     (message (format "compiling: ~a" path))
-                     ;; use #f to signal compile error. Otherwise allow errors to pass.
-                     (define result (with-handlers ([exn:fail? (λ _ (message (format "compile failed: ~a" path-name)) #f)])
-                                      (path->hash path)))
-                     (place-channel-put ch result)))
-    (place-channel-put p path)
-    p)
+  (define uncached-paths (filter-not path-cached? paths-that-should-be-cached))
   
   ;; compile the paths in groups, so they can be incrementally saved.
   ;; that way, if there's a failure, the progress is preserved.
   ;; but the slowest file in a group will prevent further progress.
   (for ([path-group (in-list (slice-at uncached-paths max-places))])
-       (define path-places (map path-into-place path-group))
+       (define path-places (map (λ (pg) (path-into-place starting-dir pg)) path-group))
        (for ([path (in-list path-group)]
              [ppl (in-list path-places)])
             (define result (place-channel-get ppl))
-            (when result ; #f is used to signal compile error
-              (cache-ref! (paths->key path) (λ _ result))))))
\ No newline at end of file
+            (when result ; #false is used to signal compile error
+              (cache-ref! (paths->key path) (λ () result))))))
\ No newline at end of file
diff --git a/pollen/private/ts.rktd b/pollen/private/ts.rktd
index 32a261a..c84f191 100644
--- a/pollen/private/ts.rktd
+++ b/pollen/private/ts.rktd
@@ -1 +1 @@
-1540858397
+1540858400