diff --git a/c/interaction/interaction.c b/c/interaction/interaction.c
index c829654..9048dee 100644
--- a/c/interaction/interaction.c
+++ b/c/interaction/interaction.c
@@ -148,10 +148,23 @@ interaction_UFuncWrapper_init
 	, PyObject * args
 	, PyObject * kwds)
 {
+	// 0: ufunc_force
+	// 1: interaction2D
 	char type;
 	PyObject * coefficients;
 
-	if(!PyArgs_ParseTupleAndKeywords(args, kwds, "" // FIXME
+	if(!PyArgs_ParseTupleAndKeywords(args, kwds, "BO!", &type, &PyList_Type, &coefficients))
+	{
+		return -1;
+	}
+
+	if(PyList_Size(coefficients) != 19)
+	{
+		PyErr_SetString("coefficients must have length 19", PyExc_ValueError);
+		return -1;
+	}
+
+
 }
 
 
@@ -167,6 +180,7 @@ static PyMethodDef InteractionMethods[] = {
 	{NULL, NULL, 0, NULL}
 };
 
+// FIXME
 PyUFuncGenericFunction interaction_funcs[] = 
 	{ &interaction_ufunc_float2D};
 PyUFuncGenericFunction force_funcs[] = 
@@ -206,7 +220,7 @@ PyInit_interaction(void)
 		return NULL;
 	}
 	import_array();
-	import_umath();
+	import_ufunc();
 
 	ufunc_interaction = PyUFunc_FromFuncAndDataAndSignature(
 			interaction_funcs