diff --git a/fslorient.cc b/fslorient.cc
index 0d87ce86fe010e480cb12f234de7b9c115a954c2..d3814f465a9fe2beadfbc88f1265557ea7b32806 100644
--- a/fslorient.cc
+++ b/fslorient.cc
@@ -50,8 +50,7 @@ void print_usage() {
 
 template <class T>
 void swaporient(volume4D<T>& invol) {
-  Matrix swapmat(4,4);
-  swapmat=Identity(4);
+  Matrix swapmat(IdentityMatrix(4));
   swapmat(1,1)=-1;
   swapmat(1,4)=invol.xsize()-1;
   invol.set_sform(invol.sform_code(),invol.sform_mat() * swapmat);