diff --git a/src/Determinants/slater_rules.irp.f b/src/Determinants/slater_rules.irp.f
index 6ece0aba..202d310d 100644
--- a/src/Determinants/slater_rules.irp.f
+++ b/src/Determinants/slater_rules.irp.f
@@ -220,8 +220,10 @@ subroutine get_double_excitation(det1,det2,exc,phase,Nint)
               ibclr(-1_bit_kind,n)+1_bit_kind ) ))
         else
           nperm = nperm + popcnt(iand(det1(k,ispin),                 &
-              ibset(0_bit_kind,m-1)-1_bit_kind)) +                   &
-              popcnt(iand(det1(j,ispin), ibclr(-1_bit_kind,n) +1_bit_kind))
+              ibset(0_bit_kind,m-1)-1_bit_kind)) 
+          if (n < bit_kind_size) then
+              nperm = nperm + popcnt(iand(det1(j,ispin), ibclr(-1_bit_kind,n) +1_bit_kind))
+          endif
           do i=j+1,k-1
             nperm = nperm + popcnt(det1(i,ispin))
           end do
@@ -246,8 +248,10 @@ subroutine get_double_excitation(det1,det2,exc,phase,Nint)
                 ibclr(-1_bit_kind,n)+1_bit_kind ) ))
           else
             nperm = nperm + popcnt(iand(det1(k,ispin),               &
-                ibset(0_bit_kind,m-1)-1_bit_kind)) +                 &
-                popcnt(iand(det1(j,ispin), ibclr(-1_bit_kind,n) +1_bit_kind))
+                ibset(0_bit_kind,m-1)-1_bit_kind)) 
+            if (n < bit_kind_size) then
+               nperm = nperm + popcnt(iand(det1(j,ispin), ibclr(-1_bit_kind,n) +1_bit_kind))
+            endif
             do l=j+1,k-1
               nperm = nperm + popcnt(det1(l,ispin))
             end do
@@ -334,9 +338,10 @@ subroutine get_mono_excitation(det1,det2,exc,phase,Nint)
         nperm = popcnt(iand(det1(j,ispin),                           &
             iand(ibset(0_bit_kind,m-1)-1_bit_kind,ibclr(-1_bit_kind,n)+1_bit_kind)))
       else
-        nperm = nperm + &
-            popcnt(iand(det1(k,ispin),ibset(0_bit_kind,m-1)-1_bit_kind)) +&
-            popcnt(iand(det1(j,ispin),ibclr(-1_bit_kind,n)+1_bit_kind))
+        nperm = nperm + popcnt(iand(det1(k,ispin),ibset(0_bit_kind,m-1)-1_bit_kind))
+        if (n < bit_kind_size) then
+            nperm = nperm + popcnt(iand(det1(j,ispin),ibclr(-1_bit_kind,n)+1_bit_kind))
+        endif
         do i=j+1,k-1
           nperm = nperm + popcnt(det1(i,ispin))
         end do