--- modules/filters/mod_filter.c	2010-04-25 21:45:29.000154000 -0500
+++ modules/filters/mod_filter.c	2010-04-25 21:47:14.000089000 -0500
@@ -135,6 +135,13 @@ 
     int err;
     ap_filter_rec_t *filter = f->frec;
 
+    /* We may get called more than once for the same filter
+     * (e.g. in a subrequest), so let's not clobber ourselves
+     */
+    if(f->ctx) {
+        return OK;
+    }
+
     harness_ctx *fctx = apr_pcalloc(f->r->pool, sizeof(harness_ctx));
     for (p = filter->providers; p; p = p->next) {
         if (p->frec->filter_init_func == filter_init) {