diff --git a/build.rs b/build.rs
index 66e16ac..b89a83c 100644
--- a/build.rs
+++ b/build.rs
@@ -2,10 +2,12 @@ use std::path::{Path, PathBuf};
 use std::{env, time};
 
 fn main() {
-    let cargo_dir = PathBuf::from(env::var("CARGO_MANIFEST_DIR").unwrap());
-    let target_dir = PathBuf::from(env::var("CARGO_BUILD_TARGET_DIR").unwrap());
+    if env::var("CARGO_BUILD_TARGET_DIR").is_ok() {
+        let cargo_dir = PathBuf::from(env::var("CARGO_MANIFEST_DIR").unwrap());
+        let target_dir = PathBuf::from(env::var("CARGO_BUILD_TARGET_DIR").unwrap());
 
-    run_cbindgen(&cargo_dir, &target_dir);
+        run_cbindgen(&cargo_dir, &target_dir);
+    }
 }
 
 fn run_cbindgen(cargo_dir: &Path, target_dir: &Path) {