diff --git a/lib/bn.js b/lib/bn.js
index b590145..855dfa6 100644
--- a/lib/bn.js
+++ b/lib/bn.js
@@ -2972,7 +2972,13 @@
     } else if (cmp > 0) {
       r.isub(this.p);
     } else {
-      r.strip();
+      if (r.strip !== undefined) {
+        // r is BN v4 instance
+        r.strip();
+      } else {
+        // r is BN v5 instance
+        r._strip();
+      }
     }
 
     return r;