diff --git a/src/pybricksMicropython/lib.test.ts b/src/pybricksMicropython/lib.test.ts
index 139f5135..f480d095 100644
--- a/src/pybricksMicropython/lib.test.ts
+++ b/src/pybricksMicropython/lib.test.ts
@@ -34,9 +34,9 @@ describe('validateFileName', () => {
         );
     });
 
-    it('should allow file names with dashes', () => {
+    it('should not allow file names with dashes', () => {
         expect(validateFileName('file-name', pythonFileExtension, [])).toBe(
-            FileNameValidationResult.IsOk,
+            FileNameValidationResult.HasInvalidCharacters,
         );
     });
 
diff --git a/src/pybricksMicropython/lib.ts b/src/pybricksMicropython/lib.ts
index 47a85fbb..1360fb6d 100644
--- a/src/pybricksMicropython/lib.ts
+++ b/src/pybricksMicropython/lib.ts
@@ -61,7 +61,7 @@ export function validateFileName(
         return FileNameValidationResult.HasInvalidFirstCharacter;
     }
 
-    if (!fileName.match(/^[a-zA-Z0-9_-]+$/)) {
+    if (!fileName.match(/^[a-zA-Z0-9_]+$/)) {
         return FileNameValidationResult.HasInvalidCharacters;
     }