--- a/virtualbox-guest-source.install
+++ b/virtualbox-guest-source.install
@@ -12,7 +12,10 @@ EOF
 # arg 1:  the new package version
 # arg 2:  the old package version
 pre_upgrade() {
-  pre_remove "$2"
+    local curver=${2%%-*}
+  # $2 is unset due to a bug. Query current version using pacman as fallback
+  [ -n "$curver" ] || curver=$(pacman -Qi virtualbox-guest-source | awk '/^Version/{print $3}')
+  pre_remove "$curver"
 }
 
 # arg 1:  the new package version
diff --git a/virtualbox-host-source.install b/virtualbox-host-source.install
index 8028cb7..1096f7b 100644
--- a/virtualbox-host-source.install
+++ b/virtualbox-host-source.install
@@ -12,7 +12,10 @@ EOF
 # arg 1:  the new package version
 # arg 2:  the old package version
 pre_upgrade() {
-  pre_remove "$2"
+  local curver=${2%%-*}
+  # $2 is unset due to a bug. Query current version using pacman as fallback
+  [ -n "$curver" ] || curver=$(pacman -Qi virtualbox-host-source | awk '/^Version/{print $3}')
+  pre_remove "$curver"
 }
 
 # arg 1:  the new package version