diff --git a/cmd/mtwebmapper/main.go b/cmd/mtwebmapper/main.go
index c1178d8..90bddcd 100644
--- a/cmd/mtwebmapper/main.go
+++ b/cmd/mtwebmapper/main.go
@@ -117,17 +117,18 @@ func main() {
 		var err error
 
 		if ifNoneMatch := r.Header.Get("If-None-Match"); ifNoneMatch != "" {
-			if etag, err = createETag(baseTile); err == nil {
-				if ifNoneMatch == etag {
-					http.Error(rw, http.StatusText(http.StatusNotModified), http.StatusNotModified)
-					return
-				}
+			if etag, err = createETag(baseTile); err != nil {
+				http.NotFound(rw, r)
+				return
+			}
+			if ifNoneMatch == etag {
+				http.Error(rw, http.StatusText(http.StatusNotModified), http.StatusNotModified)
+				return
 			}
 		}
 		img := common.LoadPNG(baseTile)
 
 		type subImage interface {
-			image.Image
 			SubImage(image.Rectangle) image.Image
 		}
 		var si subImage