diff --git a/src/math.f90 b/src/math.f90
index a1bc01a..4075194 100644
--- a/src/math.f90
+++ b/src/math.f90
@@ -74,7 +74,7 @@ contains
          print*,'catand, answer lt half precision, z**2 close to -1'
 !
       end if
-      xans = 0.5_wp_*atan2(2.0_wp_*x, one)
+      xans = 0.5_wp_*atan2(2.0_wp_*x, one-r2)
       yans = 0.25_wp_*log((r2+2.0_wp_*y+one)/(r2-2.0_wp_*y+one))
       catand = cmplx(xans, yans, wp_)
 !