diff --git a/build b/build
index e17653a5fcf586b2c3ab814e3d424483bfe4daba..f0cbd45ae77ca0a7a3e195041cf6b4674b9af658 100755
--- a/build
+++ b/build
@@ -6,7 +6,19 @@
 
 current_testing=buster
 
-srcpkg=$(dpkg-parsechangelog -S Source)
+if [ -z "$exp_srcpkg" ]
+then
+	echo "Do not call this script directly, use the build-thunderbird or build-firefox scripts"
+	exit 1
+fi
+
+srcpkg=$(dpkg-parsechangelog -S Source 2> /dev/null)
+
+if [ -z "$srcpkg" ]
+then
+	echo "This script is only meant to be run from a Debian source directory"
+	exit 1
+fi
 
 # Check that package matches called script
 if [ "$srcpkg" != $exp_srcpkg ]