diff --git a/src/black/linegen.py b/src/black/linegen.py
index 712c3c4b3c5..cb5331b4f92 100644
--- a/src/black/linegen.py
+++ b/src/black/linegen.py
@@ -273,7 +273,7 @@ def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
 
             if is_multiline_string(leaf):
                 indent = " " * 4 * self.current_line.depth
-                docstring = fix_docstring(docstring, indent, self.mode.preview)
+                docstring = fix_docstring(docstring, indent, preview=self.mode.preview)
             else:
                 docstring = docstring.strip()
 
diff --git a/src/black/strings.py b/src/black/strings.py
index e10e94d15e7..427d9ddf5f6 100644
--- a/src/black/strings.py
+++ b/src/black/strings.py
@@ -60,7 +60,7 @@ def lines_with_leading_tabs_expanded(s: str) -> List[str]:
     return lines
 
 
-def fix_docstring(docstring: str, prefix: str, preview: bool) -> str:
+def fix_docstring(docstring: str, prefix: str, *, preview: bool) -> str:
     # https://www.python.org/dev/peps/pep-0257/#handling-docstring-indentation
     if not docstring:
         return ""