diff --git a/tools/unix/build_omim.sh b/tools/unix/build_omim.sh
index b7b91ecf2..fc912f37b 100755
--- a/tools/unix/build_omim.sh
+++ b/tools/unix/build_omim.sh
@@ -2,9 +2,9 @@
 
 set -euo pipefail
 
-SKIP_MAP_DOWNLOAD=$SKIP_MAP_DOWNLOAD
-SKIP_GENERATE_SYMBOLS=$SKIP_GENERATE_SYMBOLS
-SKIP_GENERATE_DRULES=$SKIP_GENERATE_DRULES
+SKIP_MAP_DOWNLOAD="${SKIP_MAP_DOWNLOAD:-}"
+SKIP_GENERATE_SYMBOLS="${SKIP_GENERATE_SYMBOLS:-}"
+SKIP_GENERATE_DRULES="${SKIP_GENERATE_DRULES:-}"
 
 OPT_DEBUG=
 OPT_RELEASE=