diff --git a/plonky2/src/iop/challenger.rs b/plonky2/src/iop/challenger.rs
index b9883a18..97d21197 100644
--- a/plonky2/src/iop/challenger.rs
+++ b/plonky2/src/iop/challenger.rs
@@ -130,7 +130,7 @@ impl<F: RichField, H: Hasher<F>> Challenger<F, H> {
     /// Absorb any buffered inputs. After calling this, the input buffer will be empty, and the
     /// output buffer will be full.
     fn duplexing(&mut self) {
-        assert!(self.input_buffer.len() < SPONGE_RATE);
+        assert!(self.input_buffer.len() <= SPONGE_RATE);
 
         // Overwrite the first r elements with the inputs. This differs from a standard sponge,
         // where we would xor or add in the inputs. This is a well-known variant, though,