diff --git a/src/sourmash/sig/__main__.py b/src/sourmash/sig/__main__.py
index 6ef46c9e2c..5710a79150 100644
--- a/src/sourmash/sig/__main__.py
+++ b/src/sourmash/sig/__main__.py
@@ -389,14 +389,18 @@ def overlap(args):
     )
     sig1 = list(sig1)
     if len(sig1) != 1:
-        notify(f"ERROR: 'sig overlap' needs exactly one signature per file; found {len(sig1)} in '{args.signature1}'")
+        notify(
+            f"ERROR: 'sig overlap' needs exactly one signature per file; found {len(sig1)} in '{args.signature1}'"
+        )
         sys.exit(-1)
     sig2 = sourmash.load_file_as_signatures(
         args.signature2, ksize=args.ksize, select_moltype=moltype
     )
     sig2 = list(sig2)
     if len(sig2) != 1:
-        notify(f"ERROR: 'sig overlap' needs exactly one signature per file; found {len(sig2)} in '{args.signature2}'")
+        notify(
+            f"ERROR: 'sig overlap' needs exactly one signature per file; found {len(sig2)} in '{args.signature2}'"
+        )
         sys.exit(-1)
 
     sig1 = sig1[0]
@@ -663,7 +667,9 @@ def subtract(args):
     )
     from_sigobj = list(from_sigobj)
     if len(from_sigobj) != 1:
-        notify(f"ERROR: 'sig subtract' needs exactly one signature per file; found {len(from_sigobj)} in '{from_sigfile}'")
+        notify(
+            f"ERROR: 'sig subtract' needs exactly one signature per file; found {len(from_sigobj)} in '{from_sigfile}'"
+        )
         sys.exit(-1)
     from_sigobj = from_sigobj[0]
 
@@ -715,7 +721,9 @@ def subtract(args):
         )
         abund_sig = list(abund_sig)
         if len(abund_sig) != 1:
-            notify(f"ERROR: 'sig subtract' needs exactly one signature per file; found {len(abund_sig)} in '{args.abundances_from}'")
+            notify(
+                f"ERROR: 'sig subtract' needs exactly one signature per file; found {len(abund_sig)} in '{args.abundances_from}'"
+            )
             sys.exit(-1)
         abund_sig = abund_sig[0]