diff --git a/aidl.h b/aidl.h
index 29f0569..e5fd4ac 100644
--- a/aidl.h
+++ b/aidl.h
@@ -48,8 +48,8 @@
                         const IoDelegate& io_delegate);
 int compile_aidl_to_java(const JavaOptions& options,
                          const IoDelegate& io_delegate);
-bool preprocess_aidl(const JavaOptions& options,
-                     const IoDelegate& io_delegate);
+int preprocess_aidl(const JavaOptions& options,
+                    const IoDelegate& io_delegate);
 
 namespace internals {