diff --git a/plugins/pycv/PythonFunction.cpp b/plugins/pycv/PythonFunction.cpp
index caacb92603..a0aa837f3f 100644
--- a/plugins/pycv/PythonFunction.cpp
+++ b/plugins/pycv/PythonFunction.cpp
@@ -105,7 +105,7 @@ PLUMED_REGISTER_ACTION(PythonFunction,"PYFUNCTION")
 void PythonFunction::registerKeywords( Keywords& keys ) {
   Function::registerKeywords( keys );
   keys.use("PERIODIC");
-  keys.addInputKeyword("compulsory","ARG","scalar","the labels of the values from which the function is calculated");
+  keys.addInputKeyword("optional","ARG","scalar","the labels of the values from which the function is calculated");
   keys.add("compulsory","IMPORT","the python file to import, containing the function");
   keys.add("compulsory","CALCULATE",PYCV_DEFAULTCALCULATE,"the function to call");
   keys.add("compulsory","INIT",PYCV_DEFAULTINIT,"the function to call during the construction method of the function");