diff --git a/setup.py b/setup.py
index 2cb3bd78871..a52711b0ed5 100644
--- a/setup.py
+++ b/setup.py
@@ -153,7 +153,7 @@ def get_extensions():
     sources = list(extension_sources)
 
     include_dirs = [this_dir]
-    python_include_dir = os.getenv('PYTHON_INCLUDE_DIR')
+    python_include_dir = os.getenv("PYTHON_INCLUDE_DIR")
     if python_include_dir is not None:
         include_dirs.append(python_include_dir)
     ext_modules = [