Fonts messed up after using a custom resource pack font

I've copied a custom resource pack into the folder, when I start Minecraft and choose the resource pack everything works fine, but after closing and restarting the client the text looks messed up (in both the menu and ingame). When I choose the default texture pack again and restart the client everything works fine again, I can choose a custom resource pack again but the same thing happens after restarting.

The resource pack used was John Smith Legacy, but it also appears to happen when using other resource packs.