diff --git a/pbjson-types/build.rs b/pbjson-types/build.rs
index 99bebcc..eed1689 100644
--- a/pbjson-types/build.rs
+++ b/pbjson-types/build.rs
@@ -8,7 +8,9 @@ type Error = Box<dyn std::error::Error>;
 type Result<T, E = Error> = std::result::Result<T, E>;
 
 fn main() -> Result<()> {
-    let root = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
+    let root = PathBuf::from(env::var("CARGO_MANIFEST_DIR").expect(
+        "The `CARGO_MANIFEST_DIR` environment variable is required to locate descriptors.bin",
+    ));
     let descriptor_path = root.join("descriptors.bin");
     println!("cargo:rerun-if-changed={}", descriptor_path.display());