diff --git a/README.rst b/README.rst
index 0b2b00412d..8459bcac2e 100644
--- a/README.rst
+++ b/README.rst
@@ -130,6 +130,9 @@ failing, e.g.::
 
     $ pip install --user twisted
 
+On OSX, if you encounter clang: error: unknown argument: '-mno-fused-madd' you
+will need to export CFLAGS=-Qunused-arguments.
+
 Running Your Homeserver
 =======================