diff --git a/bin/_blackbox_common.sh b/bin/_blackbox_common.sh
index c227136..5feba0e 100755
--- a/bin/_blackbox_common.sh
+++ b/bin/_blackbox_common.sh
@@ -28,7 +28,7 @@ source "${0%/*}"/_stack_lib.sh
 function physical_directory_of() {
   local d=$(dirname "$1")
   local f=$(basename "$1")
-  (cd "$d" && echo "$(pwd -P)/$f" )
+  (cd "$d" && echo "$(pwd -P | sed 's/\/$//')/$f" )
 }
 
 # Set REPOBASE to the top of the repository