previewHeight changes

I have a script to sync up preview tab height across a font, but I noticed every time I ran it, the preview height just got bigger. Check this script out, it should keep it the same height right? But it gets bigger:

import GlyphsApp
thisTab = Glyphs.font.currentTab
currentPreviewHeight = thisTab.previewHeight
thisTab.previewHeight = currentPreviewHeight

Edit: if I even just run this Glyphs.font.currentTab.previewHeight = Glyphs.font.currentTab.previewHeight it keeps getting bigger?

any ideas?

I fixed it (i hope).