diff --git a/tools/testing/selftests/firmware/fw_filesystem.sh b/tools/testing/selftests/firmware/fw_filesystem.sh
index 17e16fcaa0cc..99d7f13a5637 100755
--- a/tools/testing/selftests/firmware/fw_filesystem.sh
+++ b/tools/testing/selftests/firmware/fw_filesystem.sh
@@ -29,9 +29,11 @@ test_finish()
 		echo "$OLD_TIMEOUT" >/sys/class/firmware/timeout
 	fi
 	if [ "$OLD_FWPATH" = "" ]; then
-		OLD_FWPATH=" "
+		# A zero-length write won't work; write a null byte
+		printf '\000' >/sys/module/firmware_class/parameters/path
+	else
+		echo -n "$OLD_FWPATH" >/sys/module/firmware_class/parameters/path
 	fi
-	echo -n "$OLD_FWPATH" >/sys/module/firmware_class/parameters/path
 	rm -f "$FW"
 	rmdir "$FWPATH"
 }