diff --git a/src/bn/mod.rs b/src/bn/mod.rs
index 213792ae..cc64d8cd 100644
--- a/src/bn/mod.rs
+++ b/src/bn/mod.rs
@@ -28,7 +28,6 @@ extern {
     fn BN_CTX_free(ctx: *mut BN_CTX);
 
     fn BN_set_word(bn: *mut BIGNUM, n: c_ulong) -> c_int;
-    fn BN_sub_word(bn: *mut BIGNUM, n: c_ulong) -> c_int;
     fn BN_set_negative(bn: *mut BIGNUM, n: c_int);
     fn BN_num_bits(bn: *mut BIGNUM) -> c_int;