diff --git a/ChangeLog b/ChangeLog
index a08ea9ae7e..159a8a5092 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -7,10 +7,12 @@ Changes in version 0.1.2.4-alpha - 2006-11-??
       don't recognize; it's probably just from a newer version of Tor.
     - Have directory authorities allow larger amounts of drift in uptime
       without replacing the server descriptor: previously, a server that
-      restarted every 30 minutes could have 48 "interesting" descriptors per
-      day.
+      restarted every 30 minutes could have 48 "interesting" descriptors
+      per day.
     - Start linking to the Tor specification and Tor reference manual
       correctly in the Windows installer.
+    - Avoid assert failure when our cached-routers file is empty on
+      startup.
 
 
 Changes in version 0.1.2.3-alpha - 2006-10-29
diff --git a/src/common/compat.c b/src/common/compat.c
index e07d3a3695..db38e757e3 100644
--- a/src/common/compat.c
+++ b/src/common/compat.c
@@ -141,6 +141,13 @@ tor_mmap_file(const char *filename)
   page_size = getpagesize();
   size += (size%page_size) ? page_size-(size%page_size) : 0;
 
+  if (!size) {
+    /* zero-length file. if we call mmap on it, we'll end up setting
+     * data to NULL below, and bad things will happen. So just fail. */
+    log_notice(LD_FS,"File \"%s\" is empty. Ignoring.",filename);
+    return NULL;
+  }
+
   string = mmap(0, size, PROT_READ, MAP_PRIVATE, fd, 0);
   if (string == MAP_FAILED) {
     close(fd);