There aren't any issues in either the Sample or Submit tests. For the directed graph with a single edge graph = {a -> b}, solve_graph('a', 'a', graph) should evaluate to true since you have already reached vertex a by starting at vertex a - there is no need to traverse any directed edge to reach the desired destination.

what do U mean not valid?

The last test is checking the case when not a valid object was passed as an argument

Think your 1.100 is the result of a parsing, but may be the argument you parsed wasn't a valid one and Description says:

function should return a string 'NaN' if the input is not a valid number

Was an issue with custom test cases. This has been fixed. Thanks.