diff --git a/cython/README.md b/cython/README.md
index a524ac04c..b9ce2f012 100644
--- a/cython/README.md
+++ b/cython/README.md
@@ -24,6 +24,8 @@ export PYTHONPATH=$PYTHONPATH:<GTSAM_CYTHON_INSTALL_PATH>
 - To install system-wide: run `make install` then navigate to `GTSAM_CYTHON_INSTALL_PATH` and run `python setup.py install`
     - (the same command can be used to install into a virtual environment if it is active)
     - note: if you don't want gtsam to install to a system directory such as `/usr/local`, pass `-DCMAKE_INSTALL_PREFIX="./install"` to cmake to install gtsam to a subdirectory of the build directory.
+    - if you run `setup.py` from the build directory rather than the installation directory, the script will warn you with the message: `setup.py is being run from an unexpected location`.
+      Before `make install` is run, not all the components of the package have been copied across, so running `setup.py` from the build directory would result in an incomplete package.
 
 UNIT TESTS
 ==========