diff --git a/src/dispersion.f90 b/src/dispersion.f90
index 0d780e4..ab2529d 100644
--- a/src/dispersion.f90
+++ b/src/dispersion.f90
@@ -212,6 +212,7 @@ subroutine warmdisp(xg,yg,mu,npl,nprf,sox,lrm,err,nprr,npri,fast,imx,ex,ey,ez)
       if (imx.lt.0) then
         err=1
         imxx=1
+        npra2=nprf**2
       else
         err=2
         exit