diff --git a/src/core/DisplayWindow.java b/src/core/DisplayWindow.java
index eea47b3..033c839 100644
--- a/src/core/DisplayWindow.java
+++ b/src/core/DisplayWindow.java
@@ -303,6 +303,7 @@ private DImage runFilters(DImage frameToFilter) {
                 System.err.println("Your filter has returned a null output.  Check your processImage method!");
                 System.exit(1);
             }
+            return output;
         }
         return frameToFilter;
     }