diff --git a/Sources/NESTOR_vacuum/bextern.f b/Sources/NESTOR_vacuum/bextern.f
index 1aba2cb..70cd7d9 100644
--- a/Sources/NESTOR_vacuum/bextern.f
+++ b/Sources/NESTOR_vacuum/bextern.f
@@ -3,6 +3,7 @@ SUBROUTINE bextern(plascur, wint, lscreen)
       USE mgrid_mod, ONLY: bvac
       USE parallel_include_module
       USE dbgout
+      USE stel_constants, ONLY: mu0
       IMPLICIT NONE
 C-----------------------------------------------
 C   D u m m y   A r g u m e n t s