diff --git a/src/config.rkt b/src/config.rkt
index 4894b95..18a5a1b 100644
--- a/src/config.rkt
+++ b/src/config.rkt
@@ -14,6 +14,8 @@
 
 (define-runtime-path here ".")
 (define (config-path str)
+  (unless (path-string? str)
+    (error 'config-path "Not given path string: ~e" str))
   (define p (if (relative-path? str)
                 (build-path here str)
                 str))
diff --git a/src/static.rkt b/src/static.rkt
index 5b99e71..bcfe7c9 100644
--- a/src/static.rkt
+++ b/src/static.rkt
@@ -49,7 +49,8 @@
     (and p (config-path p))))
 
 (define pkg-index-generated-directory
-  (config-path (@ (config) pkg-index-generated-directory)))
+  (config-path (or (@ (config) pkg-index-generated-directory)
+                   (error 'pkg-index-generated-directory "Not specified"))))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Static rendering daemon -- Interface
@@ -260,4 +261,4 @@
 (define (extra-files-paths)
   (list static-generated-directory
         (config-path "../static")
-        (config-path pkg-index-generated-directory)))
+        pkg-index-generated-directory))