diff --git a/src/rustint/core.clj b/src/rustint/core.clj
index 502c09e..3852640 100644
--- a/src/rustint/core.clj
+++ b/src/rustint/core.clj
@@ -2545,13 +2545,10 @@
 )
 
 (defn replace-groups [string groups]
-  (if (empty? groups)
-    string
-    (
-      replace-groups
-        (clojure.string/replace-first string #"\{:?(.\d+)?\}" (first groups))
-        (rest groups)
-    )
+  (cond
+    (nil? groups) string
+    (empty? groups) string
+    :else (replace-groups (clojure.string/replace-first string #"\{:?(.\d+)?\}" (first groups)) (rest groups))
   )
 )