diff --git a/.githooks/pre-commit b/.githooks/pre-commit
index be06c22512f373086fb851a4fcb0aa51e96067b0..24801104fe15ab56239340683595f98a1b56a9ef 100755
--- a/.githooks/pre-commit
+++ b/.githooks/pre-commit
@@ -40,6 +40,7 @@ abuild_local_sources() {
 	set -eu
 
 	status=0
+	: ${source:=""}
 	for src in $source; do
 		# Skip remote sources.
 		case "$src" in */*) continue;; esac