diff --git a/common/rfb/ScaleFilters.cxx b/common/rfb/ScaleFilters.cxx
index 28fa24f..4782add 100644
--- a/common/rfb/ScaleFilters.cxx
+++ b/common/rfb/ScaleFilters.cxx
@@ -67,7 +67,7 @@
 //
 
 SFilter &ScaleFilters::operator[](unsigned int filter_id) {
-  assert(filter_id < scaleFilterMaxNumber);
+  assert(filter_id <= scaleFilterMaxNumber);
   return filters[filter_id];
 }