diff --git a/src/lib.rs b/src/lib.rs
index d5b540a..0bf6fbc 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1647,7 +1647,7 @@ impl<B: BitBlock> BitVec<B> {
     pub fn insert(&mut self, at: usize, bit: bool) {
         assert!(
             at <= self.nbits,
-            "insertion index (is {at}) should be <= nbits (is {nbits})",
+            "insertion index (is {at}) should be <= len (is {nbits})",
             nbits = self.nbits
         );