diff --git a/clinicadl/utils/iotools/clinica_utils.py b/clinicadl/utils/iotools/clinica_utils.py
index 68426fa8e..ef6fb4a00 100644
--- a/clinicadl/utils/iotools/clinica_utils.py
+++ b/clinicadl/utils/iotools/clinica_utils.py
@@ -35,13 +35,14 @@ class FileType(BaseModel):
     needed_pipeline: Optional[str] = None
 
     @field_validator("pattern", mode="before")
-    def check_pattern(self, v):
+    def check_pattern(cls, v):
         if v[0] == "/":
             raise ValueError(
                 "pattern argument cannot start with char: / (does not work in os.path.join function). "
                 "If you want to indicate the exact name of the file, use the format "
                 "directory_name/filename.extension or filename.extension in the pattern argument."
             )
+        return v
 
 
 class FileReader(BaseModel):