Cannot walk from slab-over-water onto farmland

I have a ditch of water source blocks where I have inserted a half-slab of wood on the top half of the source blocks.  Immediately next to it I have farmland (i.e. grass blocks which have been treated with a hoe.)

What I expected to happen was...:
When I stand on the wooden half-slab and move toward the farmland, I expect to walk straight forward (being as the farmland and the wooden half-slab are on the same level.)

What actually happened was...:
I am unable to move forward.  If I jump, I can progress - but of course it's neither convenient to do so, nor cool as jumping on farmland turns it back into regular soil.