diff --git a/Geo/SVector3.h b/Geo/SVector3.h
index 97683d7b094c6921f8eda20df7ffaa7cfb609001..a8cc7dd84d6acc068027dce684a988e6d6f29831 100644
--- a/Geo/SVector3.h
+++ b/Geo/SVector3.h
@@ -91,7 +91,7 @@ class SVector3 {
   }
   const double* data() const {return P.data();}
   double* data() {return P.data();}
-  double axpy(const double a, const SVector3& y){
+  void axpy(const double a, const SVector3& y){
     for (int i=0; i<3; i++){
       P[i] += (a*y[i]);
     }