diff --git a/stackrc b/stackrc
index 887d4b3..8638a90 100644
--- a/stackrc
+++ b/stackrc
@@ -15,7 +15,7 @@
 export LC_ALL
 
 # Make tracing more educational
-export PS4='+ ${BASH_SOURCE}:${FUNCNAME[0]}:L${LINENO}:   '
+export PS4='+ ${BASH_SOURCE:-}:${FUNCNAME[0]:-}:L${LINENO:-}:   '
 
 # Find the other rc files
 RC_DIR=$(cd $(dirname "${BASH_SOURCE:-$0}") && pwd)