diff --git a/README.md b/README.md
index 22fb636..00bc797 100644
--- a/README.md
+++ b/README.md
@@ -34,11 +34,11 @@ mymc+ is available on [PyPI](https://pypi.org/project/mymcplus/).
 You can install it, including the GUI, using pip:
 
 ```
-pip install mymcplus[GUI]
+pip install mymcplus[gui]
 ```
 
-If you only wish to install the command line interface, simple omit the
-GUI extra:
+If you only wish to install the command line interface, simply omit the
+gui extra:
 
 ```
 pip install mymcplus
diff --git a/setup.py b/setup.py
index aec638c..139b7d9 100644
--- a/setup.py
+++ b/setup.py
@@ -68,6 +68,6 @@
     python_requires=">=3.4",
     install_requires=[],
     extras_require={
-        "GUI": ["wxPython", "pyopengl"]
+        "gui": ["wxPython", "pyopengl"]
     }
 )