diff --git a/runtests.sh b/runtests.sh
index 48b9858315b..d5b1c3531c6 100755
--- a/runtests.sh
+++ b/runtests.sh
@@ -22,6 +22,9 @@ CYGWIN*) PYTHON=./python.exe;;
 *)      PYTHON=./python;;
 esac
 
+# Unset PYTHONPATH, just to be sure.
+unset PYTHONPATH
+
 # Create the output directory if necessary.
 mkdir -p OUT