diff --git a/src/dispersion.f90 b/src/dispersion.f90
index 9081bbe..8ef76ea 100644
--- a/src/dispersion.f90
+++ b/src/dispersion.f90
@@ -1190,7 +1190,7 @@ end subroutine fsup
 !     this work published in transactions on mathematical software,
 !     vol. 16, no. 1, pp. 47.
 !
-  subroutine zetac (xi, yi, zr, zi, iflag)
+  pure subroutine zetac (xi, yi, zr, zi, iflag)
 !
 !  given a complex number z = (xi,yi), this subroutine computes
 !  the value of the faddeeva-function w(z) = exp(-z**2)*erfc(-i*z),