diff --git a/modules/cache/cache_util.c b/modules/cache/cache_util.c
index 7b7fb45..fbebb1e 100644
--- a/modules/cache/cache_util.c
+++ b/modules/cache/cache_util.c
@@ -1251,8 +1251,10 @@ CACHE_DECLARE(apr_table_t *)ap_cache_cacheable_headers_out(request_rec *r)
 
     if (!apr_table_get(headers_out, "Content-Type")
         && r->content_type) {
-        apr_table_setn(headers_out, "Content-Type",
-                       ap_make_content_type(r, r->content_type));
+        const char *ctype = ap_make_content_type(r, r->content_type);
+        if (ctype) {
+            apr_table_setn(headers_out, "Content-Type", ctype);
+        }
     }
 
     if (!apr_table_get(headers_out, "Content-Encoding")
