--- acpi.c	2009-03-23 15:23:12.000000000 +0000
+++ acpi.c.patched	2009-11-05 09:38:25.000000000 +0000
@@ -303,6 +303,9 @@
 		state = value->value;
 	    } else if (!strcmp(value->attr, "voltage_now")) {
 		voltage = get_unit_value(value->value) / 1000;
+		if (voltage == 0 ) {
+			voltage = -1;
+		}
 	    }
 	    fields = list_next(fields);
 	}