diff --git a/qtpy/__init__.py b/qtpy/__init__.py
index f4fd0b6d..d0ce4449 100644
--- a/qtpy/__init__.py
+++ b/qtpy/__init__.py
@@ -72,6 +72,10 @@ class PythonQtWarning(Warning):
     """Warning if some features are not implemented in a binding."""
 
 
+class PythonQtValueError(ValueError):
+    """Error raised if an invalid QT_API is specified."""
+
+
 # Qt API environment variable name
 QT_API = 'QT_API'
 
@@ -98,14 +102,14 @@ class PythonQtWarning(Warning):
 # Detecting if a binding was specified by the user
 binding_specified = QT_API in os.environ
 
-# Setting a default value for QT_API
-os.environ.setdefault(QT_API, 'pyqt5')
-
 API_NAMES = {'pyqt5': 'PyQt5', 'pyqt6': 'PyQt6',
-             'pyside2':'PySide2', 'pyside6': 'PySide6'}
-API = os.environ[QT_API].lower()
+             'pyside2': 'PySide2', 'pyside6': 'PySide6'}
+API = os.environ.get(QT_API, 'pyqt5').lower()
 initial_api = API
-assert API in API_NAMES
+if API not in API_NAMES:
+    raise PythonQtValueError(
+        f'Specified QT_API={repr(QT_API.lower())} is not in valid options: '
+        f'{API_NAMES}')
 
 is_old_pyqt = is_pyqt46 = False
 QT5 = PYQT5 = True
@@ -150,7 +154,9 @@ class PythonQtWarning(Warning):
 
             del macos_version
     except ImportError:
-        API = os.environ['QT_API'] = 'pyqt6'
+        API = 'pyqt6'
+    else:
+        os.environ[QT_API] = API
 
 if API in PYQT6_API:
     try:
@@ -161,7 +167,9 @@ class PythonQtWarning(Warning):
         QT6 = PYQT6 = True
 
     except ImportError:
-        API = os.environ['QT_API'] = 'pyside2'
+        API = 'pyside2'
+    else:
+        os.environ[QT_API] = API
 
 
 if API in PYSIDE2_API:
@@ -183,7 +191,9 @@ class PythonQtWarning(Warning):
 
             del macos_version
     except ImportError:
-        API = os.environ['QT_API'] = 'pyside6'
+        API = 'pyside6'
+    else:
+        os.environ[QT_API] = API
 
 if API in PYSIDE6_API:
     try:
@@ -194,7 +204,9 @@ class PythonQtWarning(Warning):
         QT6 = PYSIDE6 = True
 
     except ImportError:
-        API = os.environ['QT_API'] = 'pyqt5'
+        API = 'pyqt5'
+    else:
+        os.environ[QT_API] = API
 
 
 # If a correct API name is passed to QT_API and it could not be found,
diff --git a/qtpy/tests/conftest.py b/qtpy/tests/conftest.py
index ea1fd190..a24f664f 100644
--- a/qtpy/tests/conftest.py
+++ b/qtpy/tests/conftest.py
@@ -21,8 +21,9 @@ def pytest_report_header(config):
     versions += 'PyQt6: '
 
     try:
-        from PyQt6 import Qt
-        versions += f"PyQt: {Qt.PYQT_VERSION_STR} - Qt: {Qt.QT_VERSION_STR}"
+        from PyQt6 import QtCore
+        versions += \
+            f"PyQt: {QtCore.PYQT_VERSION_STR} - Qt: {QtCore.QT_VERSION_STR}"
     except ImportError:
         versions += 'not installed'
     except AttributeError:
diff --git a/qtpy/tests/test_main.py b/qtpy/tests/test_main.py
index 86aeb5d2..000ddf10 100644
--- a/qtpy/tests/test_main.py
+++ b/qtpy/tests/test_main.py
@@ -1,6 +1,11 @@
 import os
+import sys
+import subprocess
+
+import pytest
+
+from qtpy import QtCore, QtGui, QtWidgets, API_NAMES, PythonQtValueError
 
-from qtpy import QtCore, QtGui, QtWidgets
 try:
     # removed in qt 6.0
     from qtpy import QtWebEngineWidgets
@@ -16,6 +21,7 @@ def assert_pyside2():
     assert QtGui.QPainter is PySide2.QtGui.QPainter
     assert QtWidgets.QWidget is PySide2.QtWidgets.QWidget
     assert QtWebEngineWidgets.QWebEnginePage is PySide2.QtWebEngineWidgets.QWebEnginePage
+    assert os.environ['QT_API'] == 'pyside2'
 
 def assert_pyside6():
     """
@@ -27,6 +33,7 @@ def assert_pyside6():
     assert QtWidgets.QWidget is PySide6.QtWidgets.QWidget
     # Only valid for qt>=6.2
     # assert QtWebEngineWidgets.QWebEnginePage is PySide6.QtWebEngineCore.QWebEnginePage
+    assert os.environ['QT_API'] == 'pyside6'
 
 def assert_pyqt5():
     """
@@ -40,6 +47,7 @@ def assert_pyqt5():
         assert QtWebEngineWidgets.QWebEnginePage is PyQt5.QtWebEngineWidgets.QWebEnginePage
     else:
         assert QtWebEngineWidgets.QWebEnginePage is PyQt5.QtWebKitWidgets.QWebPage
+    assert os.environ['QT_API'] == 'pyqt5'
 
 def assert_pyqt6():
     """
@@ -49,6 +57,7 @@ def assert_pyqt6():
     assert QtCore.QEvent is PyQt6.QtCore.QEvent
     assert QtGui.QPainter is PyQt6.QtGui.QPainter
     assert QtWidgets.QWidget is PyQt6.QtWidgets.QWidget
+    assert os.environ['QT_API'] == 'pyqt6'
 
 
 def test_qt_api():
@@ -83,3 +92,39 @@ def test_qt_api():
                 assert_pyqt6()
         else:
             assert_pyqt5()
+
+
+@pytest.mark.parametrize('api', API_NAMES.values())
+def test_qt_api_environ(api):
+    """
+    If no QT_API is specified but some Qt is imported, ensure QT_API is set properly.
+    """
+    mod = f'{api}.QtCore'
+    pytest.importorskip(mod, reason=f'Requires {api}')
+    # clean env
+    env = os.environ.copy()
+    for key in ('QT_API', 'USE_QT_API'):
+        if key in env:
+            del env[key]
+    cmd = f"""
+import {mod}
+from qtpy import API
+import os
+print(API)
+print(os.environ['QT_API'])
+"""
+    output = subprocess.check_output([sys.executable, '-c', cmd], env=env)
+    got_api, env_qt_api = output.strip().decode('utf-8').splitlines()
+    assert got_api == api.lower()
+    assert env_qt_api == api.lower()
+    # Also ensure we raise a nice error
+    env['QT_API'] = 'bad'
+    cmd = """
+try:
+    import qtpy
+except ValueError as exc:
+    assert 'Specified QT_API' in str(exc), str(exc)
+else:
+    raise AssertionError('QtPy imported despite bad QT_API')
+"""
+    subprocess.check_call([sys.executable, '-Oc', cmd], env=env)