diff --git a/qtpy/__init__.py b/qtpy/__init__.py
index b6392f5d..b8373439 100644
--- a/qtpy/__init__.py
+++ b/qtpy/__init__.py
@@ -68,7 +68,7 @@
 PYSIDE_API = ['pyside']
 
 os.environ.setdefault(QT_API, 'pyqt5')
-API = os.environ[QT_API]
+API = os.environ[QT_API].lower()
 assert API in (PYQT5_API + PYQT4_API + PYSIDE_API)
 
 is_old_pyqt = is_pyqt46 = False