diff --git a/bin/dib-run-parts b/bin/dib-run-parts
index af5fde9..50ff8dc 100755
--- a/bin/dib-run-parts
+++ b/bin/dib-run-parts
@@ -100,7 +100,7 @@ for target in $(find . -name 'start_*' -printf '%f\n') ; do
     stop_file=stop_${target##start_}
     start_seconds=$(cat $target)
     stop_seconds=$(cat $stop_file)
-    duration=$(python -c "print $stop_seconds - $start_seconds")
+    duration=$(python -c "print($stop_seconds - $start_seconds)")
     LC_NUMERIC=C printf "%-40s %10.3f\n" ${target##start_} $duration
 done
 popd > /dev/null