diff --git a/INSTALL.md b/INSTALL.md
index 3fe4eed..3e2d626 100644
--- a/INSTALL.md
+++ b/INSTALL.md
@@ -25,7 +25,7 @@ and install all dependencies in a venv in the mangle directory, e.g.
 ```
 ...\mangle> virtualenv venv
 > venv\Scripts\activate
-(venv) > pip install requirements.txt
+(venv) > pip install -r requirements.txt
 ```
 
 You can run the GUI via
diff --git a/requirements.txt b/requirements.txt
index 0b36984..dac81c4 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,4 +1,4 @@
 Pillow==6.2.2
 py2exe-py2==0.6.9
-PyQt4 @ https://download.lfd.uci.edu/pythonlibs/w4tscw6k/cp27/PyQt4-4.11.4-cp27-cp27m-win32.whl
+PyQt4 @ https://download.lfd.uci.edu/pythonlibs/w4tscw6k/cp27/PyQt4-4.11.4-cp27-cp27m-win32.whl; platform_system=='Windows'
 reportlab==3.5.59