diff --git a/xwords4/android/scripts/getsigs.sh b/xwords4/android/scripts/getsigs.sh
index fd8d6d6fe..22c05da91 100755
--- a/xwords4/android/scripts/getsigs.sh
+++ b/xwords4/android/scripts/getsigs.sh
@@ -1,9 +1,10 @@
-#!/bin/sh
+#!/bin/bash
 
 set -e -u
 
 DIR=""
 NODE=""
+CLASSPATH=${CLASSPATH:-""}
 
 usage() {
     [ $# -gt 0 ] && echo "Error: $1"
@@ -12,6 +13,11 @@ usage() {
     exit 1
 }
 
+# Make sure android.jar is on CLASSPATH
+if [ "${CLASSPATH/android.jar/}" = "$CLASSPATH" ]; then
+	usage "android.jar not on CLASSPATH"
+fi
+
 while [ $# -gt 1 ]; do
     case $1 in
         --dir)