diff --git a/Cargo.toml b/Cargo.toml
index 57f81fa..c9cb6d5 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -16,3 +16,4 @@ readme = "README.md"
 [features]
 default = ["std"]
 std = []
+i128 = []
diff --git a/ci/test_full.sh b/ci/test_full.sh
index c86c178..adfe4f2 100755
--- a/ci/test_full.sh
+++ b/ci/test_full.sh
@@ -11,3 +11,7 @@ cargo test --verbose
 # test `no_std`
 cargo build --verbose --no-default-features
 cargo test --verbose --no-default-features
+
+# test `i128`
+cargo build --verbose --features=i128
+cargo test --verbose --features=i128