diff --git a/mriqc/interfaces/bids.py b/mriqc/interfaces/bids.py
index f83e0b9e..a2de5227 100644
--- a/mriqc/interfaces/bids.py
+++ b/mriqc/interfaces/bids.py
@@ -44,7 +44,7 @@ class IQMFileSinkInputSpec(DynamicTraitedSpec, BaseInterfaceInputSpec):
     in_file = Str(mandatory=True, desc='path of input file')
     modality = Str(mandatory=True, desc='the qc type')
     entities = traits.Dict(desc='entities corresponding to the input')
-    subject_id = Str(mandatory=True, desc='the subject id')
+    subject_id = Str(desc='the subject id')
     session_id = traits.Either(None, Str, usedefault=True)
     task_id = traits.Either(None, Str, usedefault=True)
     acq_id = traits.Either(None, Str, usedefault=True)