diff --git a/README.md b/README.md
index e2fe808..07487a2 100644
--- a/README.md
+++ b/README.md
@@ -28,7 +28,7 @@ $ cargo install brainease
 <br />
 
 ```r
-# brainease -f examples/hello.bz
+# brainease -f examples/hello.brain
 
 save 'H' at *1
 save 'e' at *2
diff --git a/src/main.rs b/src/main.rs
index df96eb3..b002415 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -16,6 +16,11 @@ fn main() {
 
   log::trace!("Cli args: {:?}", args);
 
+  if !args.main.ends_with(".brain") {
+    log::error!("{} does not end with .brain", args.main);
+    process::exit(1);
+  }
+
   let (absolute_path, filename) = util::normalize_path(&args.main);
 
   log::trace!("Reading '{}'", filename);
diff --git a/tests/integration_tests.rs b/tests/integration_tests.rs
index 4eecf7e..d17cd99 100644
--- a/tests/integration_tests.rs
+++ b/tests/integration_tests.rs
@@ -79,6 +79,10 @@ fn run_resources() -> Result<()> {
   let files = scan_dir(&Path::new("tests/resources").to_path_buf())?;
 
   for (filename, content) in files {
+    if !filename.ends_with(".brain") {
+      continue;
+    }
+
     let instructions = parser::parse_file(&content);
     let io_handler = TestIoHandler::build_from_file(content.lines().collect());
 
diff --git a/tests/resources/goto-directions.bz b/tests/resources/goto-directions.brain
similarity index 100%
rename from tests/resources/goto-directions.bz
rename to tests/resources/goto-directions.brain
diff --git a/tests/resources/hello.bz b/tests/resources/hello.brain
similarity index 100%
rename from tests/resources/hello.bz
rename to tests/resources/hello.brain
diff --git a/tests/resources/if-simple-test.bz b/tests/resources/if-simple-test.brain
similarity index 100%
rename from tests/resources/if-simple-test.bz
rename to tests/resources/if-simple-test.brain
diff --git a/tests/resources/loop.bz b/tests/resources/loop.brain
similarity index 100%
rename from tests/resources/loop.bz
rename to tests/resources/loop.brain