diff --git a/install.sh b/install.sh
index 2e832ad..df950ae 100755
--- a/install.sh
+++ b/install.sh
@@ -468,7 +468,11 @@ is_host_amd64_elf() {
 # Test if a location is writeable by trying to write to it. Windows does not let
 # you test writeability other than by writing: https://stackoverflow.com/q/1999988
 test_writeable() {
-    path="${1:-}/test.txt"
+    if [ -z "${1-}" ]; then
+        # There's a bug if this is reached.
+        abort "BUG: test_writeable requires a path to test."
+    fi
+    path="$1/test.txt"
     if touch "${path}" 2>/dev/null; then
         rm "${path}"
         return 0