diff --git a/setup_support.py b/setup_support.py
index d1fbf3d50..4cab17f33 100644
--- a/setup_support.py
+++ b/setup_support.py
@@ -66,7 +66,7 @@ def _find_lib():
         ffi.dlopen('secp256k1')
         include_path = subprocess.check_output(['pkg-config', '--static', '--cflags-only-I', 'libsecp256k1'])  # noqa S603
         include_path = include_path.decode('UTF-8').strip().split()[0][2:]
-        return os.path.exists(f'{include_path}/secp256k1_ecdh.h')
+        return os.path.exists(os.path.join(include_path, 'secp256k1_ecdh.h'))
     except OSError:
         if 'LIB_DIR' in os.environ:
             for path in glob.glob(os.path.join(os.environ['LIB_DIR'], '*secp256k1*')):