diff --git a/Modules/getpath.c b/Modules/getpath.c
index 10ad509157a..418c683ce66 100644
--- a/Modules/getpath.c
+++ b/Modules/getpath.c
@@ -190,10 +190,14 @@ isdir(char *filename)                   /* Is directory */
 }
 
 
-/* joinpath requires that any buffer argument passed to it has at
-   least MAXPATHLEN + 1 bytes allocated.  If this requirement is met,
-   it guarantees that it will never overflow the buffer.  If stuff
-   is too long, buffer will contain a truncated copy of stuff.
+/* Add a path component, by appending stuff to buffer.
+   buffer must have at least MAXPATHLEN + 1 bytes allocated, and contain a
+   NUL-terminated string with no more than MAXPATHLEN characters (not counting
+   the trailing NUL).  It's a fatal error if it contains a string longer than
+   that (callers must be careful!).  If these requirements are met, it's
+   guaranteed that buffer will still be a NUL-terminated string with no more
+   than MAXPATHLEN characters at exit.  If stuff is too long, only as much of
+   stuff as fits will be appended.
 */
 static void
 joinpath(char *buffer, char *stuff)
@@ -206,6 +210,8 @@ joinpath(char *buffer, char *stuff)
         if (n > 0 && buffer[n-1] != SEP && n < MAXPATHLEN)
             buffer[n++] = SEP;
     }
+    if (n > MAXPATHLEN)
+    	Py_FatalError("buffer overflow in getpath.c's joinpath()");
     k = strlen(stuff);
     if (n + k > MAXPATHLEN)
         k = MAXPATHLEN - n;