Hey community! Since a couple of hours I've been dealing with the question why the decay $$D^0 \rightarrow K^+ K^-$$ is suppressed. If I draw the Feynman diagram for the first order decay, everything seems pretty alright as the charm quark can couple with a strange quark. The resulting W+ can then produce an up and anti-strange quark which gives above K mesons. What piece am I missing?