diff --git a/include/http_core.h b/include/http_core.h
index 92bf161ce2..3ecd5f119f 100644
--- a/include/http_core.h
+++ b/include/http_core.h
@@ -725,7 +725,7 @@ typedef struct {
     apr_array_header_t *protocols;
     int protocols_honor_order;
     int async_filter;
-    int async_filter_set:1;
+    unsigned int async_filter_set:1;
 } core_server_config;
 
 /* for AddOutputFiltersByType in core.c */