diff --git a/hyperpoint.cpp b/hyperpoint.cpp
index 185ccca7..4afac8d0 100644
--- a/hyperpoint.cpp
+++ b/hyperpoint.cpp
@@ -877,6 +877,14 @@ EX void orthonormalize(transmatrix& T) {
     }
   }
 
+/** fix a 3D rotation matrix */
+EX void fix_rotation(transmatrix& rot) {
+  dynamicval<eGeometry> g(geometry, gSphere); 
+  fixmatrix(rot); 
+  for(int i=0; i<3; i++) rot[i][3] = rot[3][i] = 0;
+  rot[3][3] = 1;
+  }
+
 /** determinant 2x2 */
 EX ld det2(const transmatrix& T) {
   return T[0][0] * T[1][1] - T[0][1] * T[1][0];
diff --git a/nonisotropic.cpp b/nonisotropic.cpp
index abd2e42b..44bca5f1 100644
--- a/nonisotropic.cpp
+++ b/nonisotropic.cpp
@@ -2791,7 +2791,7 @@ EX namespace nisot {
     transmatrix push = eupush( tC0(T) );
     transmatrix push_back = eupush(tC0(T), -1);
     transmatrix gtl = push_back * T;
-    { dynamicval<eGeometry> g(geometry, gSphere); hr::fixmatrix(gtl); }
+    fix_rotation(gtl);
     T = push * gtl;
     }
 
diff --git a/shmup.cpp b/shmup.cpp
index dbfddd21..42729cb8 100644
--- a/shmup.cpp
+++ b/shmup.cpp
@@ -176,7 +176,7 @@ void fix_to_2(transmatrix& T) {
   if(nonisotropic) {
     hyperpoint h = tC0(T);
     transmatrix rot = gpushxto0(h) * T;
-    { dynamicval<eGeometry> g(geometry, gSphere); fixmatrix(rot); }
+    fix_rotation(rot);
     T = rgpushxto0(h) * rot;
     }
   else