diff --git a/hgext/fix.py b/hgext/fix.py
--- a/hgext/fix.py
+++ b/hgext/fix.py
@@ -540,12 +540,14 @@
         May return None if there is no appropriate command to run for the given
         parameters.
         """
-        parts = [self._command.format(rootpath=path,
-                                      basename=os.path.basename(path))]
+        parts = [pycompat.sysstr(self._command).format(
+            rootpath=path,
+            basename=os.path.basename(path))]
         if self._linerange:
             if not ranges:
                 # No line ranges to fix, so don't run the fixer.
                 return None
             for first, last in ranges:
-                parts.append(self._linerange.format(first=first, last=last))
-        return ' '.join(parts)
+                parts.append(pycompat.sysstr(self._linerange).format(
+                    first=first, last=last))
+        return ' '.join(pycompat.sysbytes(b) for b in parts)