Problem entering negative numbers to settings in GCM (Android & iOS)

I have some app settings which are set as "numeric" type and allow users to enter any number, but there can be difficulty entering a negative number into the setting, specifically in entering the negative sign itself.

1) In GCM Android if you delete the existing value from a numeric setting and start typing a negative number starting with the "-" then it won't let you enter the "-" on its own. First you have to type a number, then move the caret back to the start of the line, and then it lets you enter the "-".

2) I've also recently had a user report the same thing on iOS (I assume GCM but they haven't confirmed it yet). They couldn't enter a negative sign whatever they tried - even trying to insert it before an existing positive number. The only workaround they could find was to copy and paste a negative number into the setting from another app.