diff --git a/cover/private/html/html.rkt b/cover/private/html/html.rkt
index a8a5fd4..6835d06 100644
--- a/cover/private/html/html.rkt
+++ b/cover/private/html/html.rkt
@@ -78,6 +78,27 @@
   (cons (list (build-path dir "index.html") dir index)
         file-list))
 
+
+;; copied from the internals for backwards compatability reasons
+(define (take-common-prefix as bs [same? equal?])
+  (let-values ([(prefix atail btail)
+                (internal-split-common-prefix as bs same? #t 'take-common-prefix)])
+    prefix))
+(define (internal-split-common-prefix as bs same? keep-prefix? name)
+  (unless (list? as)
+    (raise-argument-error name "list?" as))
+  (unless (list? bs)
+    (raise-argument-error name "list?" bs))
+  (unless (and (procedure? same?)
+               (procedure-arity-includes? same? 2))
+    (raise-argument-error name "(any/c any/c . -> . any/c)" same?))
+  (let loop ([as as] [bs bs])
+    (if (and (pair? as) (pair? bs) (same? (car as) (car bs)))
+        (let-values ([(prefix atail btail) (loop (cdr as) (cdr bs))])
+          (values (and keep-prefix? (cons (car as) prefix)) atail btail))
+        (values null as bs))))
+
+
 (module+ test
   (test-begin
    (parameterize ([current-directory root]