Map a Number Item to a Slider

A simple Item that is maped to slider always casues a exception

java.lang.IllegalArgumentException: Value must be between 0 and 100

Here ist the item:
Number WZ1OG_Licht

There is the sitemap:
Slider item=WZ1OG_Licht label=“Licht” icon=“light”

I also tried to set the value on startup in a rule:
rule "Init"
when
System started
then
WZ1OG_Licht.postUpdate(1)
end

which is sometimes succsessfull but not in a stable way. If i reopen the sitemap, I get the error in the log file and the hole site failed to load.
HTTP ERROR 500
Problem accessing /basicui/app. Reason:

Server Error

Caused by:
java.lang.IllegalArgumentException: Value must be between 0 and 100
at org.eclipse.smarthome.core.library.types.PercentType.validateValue(PercentType.java:50)
at org.eclipse.smarthome.core.library.types.PercentType.(PercentType.java:45)
at org.eclipse.smarthome.core.library.types.DecimalType.as(DecimalType.java:153)
at org.eclipse.smarthome.core.items.GenericItem.getStateAs(GenericItem.java:87)
at org.eclipse.smarthome.ui.internal.items.ItemUIRegistryImpl.convertState(ItemUIRegistryImpl.java:485)
at org.eclipse.smarthome.ui.internal.items.ItemUIRegistryImpl.getState(ItemUIRegistryImpl.java:464)
at org.eclipse.smarthome.ui.basic.internal.render.AbstractWidgetRenderer.getState(Abstract

BTW: I will process the value of the slider in an other rule to do something, but this is not the part of the question.

Openhab 2.1
Raspi 3

Please use Dimmer item type instead