Pike v7.8 release 350 running Hilfe v3.5 (Incremental Pike Frontend)

class c { int foo() { constant a = 17<<1; return a; } }; class c { int foo() { constant a = 17<<0; return a; } };

Compiler Error: 1: Constant definition is not constant.

Excuse me? Why is 17<<0 less constant than 17<<1?