diff --git a/ALLCools/utilities.py b/ALLCools/utilities.py
index c8ab459..27e16e6 100644
--- a/ALLCools/utilities.py
+++ b/ALLCools/utilities.py
@@ -76,8 +76,8 @@ def parse_mc_pattern(pattern: str) -> set:
         except KeyError:
             raise KeyError(f"Base {base} is not in IUPAC table.")
     context_set = {"".join(i) for i in itertools.product(*all_pos_list)}
-    if pattern == 'CH':
-        context_set.add('CH')
+    if pattern == "CH":
+        context_set.add("CH")
     return context_set