diff --git a/build_kzip.bash b/build_kzip.bash
index 22ff50453..c1d09f751 100755
--- a/build_kzip.bash
+++ b/build_kzip.bash
@@ -6,13 +6,19 @@
 # The following environment variables affect the result:
 #   TARGET_PRODUCT        target device name, e.g., 'aosp_blueline'
 #   TARGET_BUILD_VARIANT  variant, e.g., `userdebug`
-#   OUT_DIR               where the build is happening (./out if not specified)
+#   OUT_DIR               absolute path  where the build is happening ($PWD/out if not specified})
 #   DIST_DIR              where the resulting all.kzip will be placed
 #   XREF_CORPUS           source code repository URI, e.g., 'android.googlesource.com/platform/superproject'
 #   BUILD_NUMBER          build number, used to generate unique ID (will use UUID if not set)
 
+# If OUT_DIR is not set, the build will use out/ as output directory, which is
+# a relative path. Make it absolute, otherwise the indexer will not know that it
+# contains only generated files.
+: ${OUT_DIR:=$PWD/out}
+[[ "$OUT_DIR" =~ ^/ ]] || { echo "$OUT_DIR is not an absolute path"; exit 1; }
+
 # The extraction might fail for some source files, so run with -k
-build/soong/soong_ui.bash --build-mode --all-modules --dir=$PWD -k merge_zips xref_cxx xref_java
+OUT_DIR=$OUT_DIR build/soong/soong_ui.bash --build-mode --all-modules --dir=$PWD -k merge_zips xref_cxx xref_java
 
 # We build with -k, so check that we have generated at least 100K files
 # (the actual number is 180K+)
@@ -22,5 +28,5 @@ declare -r kzip_count=$(find $OUT_DIR -name '*.kzip' | wc -l)
 # Pack
 # TODO(asmundak): this should be done by soong.
 declare -r allkzip=all.kzip
-"${OUT_DIR:-out}/soong/host/linux-x86/bin/merge_zips" "$DIST_DIR/$allkzip" @<(find $OUT_DIR -name '*.kzip')
+"$OUT_DIR/soong/host/linux-x86/bin/merge_zips" "$DIST_DIR/$allkzip" @<(find $OUT_DIR -name '*.kzip')
 echo "${BUILD_NUMBER:-$(uuidgen)}" >"$DIST_DIR/revision.txt"