64: non-mandatory-script-verify-flag (No error) (-26) What does this error mean?

I've manually constructed a transaction, and I get this error when I try to insert it in to the network.
From the source code:

// Check whether the failure was caused by a
// non-mandatory script verification check, such as
// non-standard DER encodings or non-null dummy
// arguments; if so, don't trigger DoS protection to
// avoid splitting the network between upgraded and
// non-upgraded nodes.


It sounds like maybe your transaction signature isn't standard.