diff --git a/src/propeller/variation.cljc b/src/propeller/variation.cljc
index 1a077a9..05f5302 100644
--- a/src/propeller/variation.cljc
+++ b/src/propeller/variation.cljc
@@ -173,6 +173,18 @@ The function `new-individual` returns a new individual produced by selection and
   (+ (* 0.5 (metrics/multiset-distance p1 p2))
      (math/abs (- (count p1) (count p2)))))
 
+(defn fill-empty-genes
+  "A utility function for bmx-related genetic operators. Returns the provided
+   plushy with any empty genes (regions before/between/after instances of :gap)
+   filled with a new random instruction."
+  [plushy instructions]
+  (flatten (interpose :gap
+                      (mapv (fn [gene]
+                              (if (empty? gene)
+                               (utils/random-instruction instructions)
+                               gene))
+                            (utils/extract-genes plushy)))))
+
 (defn bmx
   "Crosses over two plushies using best match crossover (bmx)."
   [plushy-a plushy-b rate max-distance argmap]
@@ -250,7 +262,8 @@ The function `new-individual` returns a new individual produced by selection and
              max-distance (utils/onenum (:bmx-maximum-distance argmap))]
          (->  (bmx plushy1 plushy2 bmx-exchange-rate max-distance argmap)
               (uniform-gap-addition gap-change-prob)
-              (uniform-gap-deletion gap-change-prob)))
+              (uniform-gap-deletion gap-change-prob)
+              (fill-empty-genes (:instructions argmap))))
        ;
        :umad ;; uniform mutation by addition and deletion, see uniform-deletion for the
                ;; adjustment that makes this size neutral on average
@@ -281,7 +294,8 @@ The function `new-individual` returns a new individual produced by selection and
               (uniform-gap-addition gap-change-prob)
               (uniform-gap-deletion gap-change-prob)
               (uniform-addition (:instructions argmap) umad-rate)
-              (uniform-deletion umad-rate)))
+              (uniform-deletion umad-rate)
+              (fill-empty-genes (:instructions argmap))))
        ;
        :rumad ;; responsive UMAD, uses a deletion rate computed from the actual
                 ;; number of additions made