diff --git a/niworkflows/interfaces/bids.py b/niworkflows/interfaces/bids.py
index ec7131dff3a..508574fb3bf 100644
--- a/niworkflows/interfaces/bids.py
+++ b/niworkflows/interfaces/bids.py
@@ -603,9 +603,7 @@ def _run_interface(self, runtime):
         if custom_entities:
             # Example: f"{key}-{{{key}}}" -> "task-{task}"
             custom_pat = '_'.join(f'{key}-{{{key}}}' for key in sorted(custom_entities))
-            patterns = [
-                pat.replace('_{suffix', '_'.join(('', custom_pat, '{suffix'))) for pat in patterns
-            ]
+            patterns = [pat.replace('_{suffix', f'_{custom_pat}_{{suffix') for pat in patterns]
 
         # Build the output path(s)
         dest_files = build_path(out_entities, path_patterns=patterns)
@@ -1108,9 +1106,7 @@ def _run_interface(self, runtime):
         if custom_entities:
             # Example: f"{key}-{{{key}}}" -> "task-{task}"
             custom_pat = '_'.join(f'{key}-{{{key}}}' for key in sorted(custom_entities))
-            patterns = [
-                pat.replace('_{suffix', '_'.join(('', custom_pat, '{suffix'))) for pat in patterns
-            ]
+            patterns = [pat.replace('_{suffix', f'_{custom_pat}_{{suffix') for pat in patterns]
 
         # Prepare SimpleInterface outputs object
         self._results['out_file'] = []
diff --git a/niworkflows/utils/spaces.py b/niworkflows/utils/spaces.py
index eb82851bd7c..ca386bcff64 100644
--- a/niworkflows/utils/spaces.py
+++ b/niworkflows/utils/spaces.py
@@ -722,7 +722,7 @@ def __call__(self, parser, namespace, values, option_string=None):
                 # relevant discussions:
                 # https://github.com/nipreps/niworkflows/pull/457#discussion_r375510227
                 # https://github.com/nipreps/niworkflows/pull/494
-                val = ':'.join((val, 'res-native'))
+                val = f'{val}:res-native'
             for sp in Reference.from_string(val):
                 spaces.add(sp)
         setattr(namespace, self.dest, spaces)