diff --git a/code/math.f90 b/code/math.f90
index 977bb3326..5a98ec0c3 100644
--- a/code/math.f90
+++ b/code/math.f90
@@ -1600,6 +1600,7 @@ pure function math_transpose3x3(A)
 
  real(pReal), dimension(4), intent(in) :: Q
  real(pReal), dimension(3) :: math_QuaternionToEuler
+ real(pReal) acos_arg
 
  math_QuaternionToEuler(2) = acos(1.0_pReal-2.0_pReal*(Q(2)*Q(2)+Q(3)*Q(3)))