I'm getting runtime errors (Specified cast is not valid) when attempting to cast Instruction.Value (for Stloc and Ldloc OpCode families) to an integer e.g.:

int variableIndex; if(instruction.OpCode == OpCode.Stloc_S) { variableIndex = (int)instruction.Value; }

An example line raising an error is
SqlDataReader dr = cmd.ExecuteReader();
which compiles to the IL:
IL_00b8: callvirt instance class [System.Data]System.Data.SqlClient.SqlDataReader [System.Data]System.Data.SqlClient.SqlCommand::ExecuteReader()
IL_00bd: stloc.s dr
According to the MSDN documentation, stloc.c has argumentindex of type int (unsigned int8, to be precise) whereas this appears to be using a string rather than a local variable index. dr is referenced in the locals section at the top of the dissembly as
[4] class [System.Data]System.Data.SqlClient.SqlDataReader dr
So I'm wondering whether a) I'm casting incorrectly or b) instruction.Value has the string indentifier, and not the declared index?
Cheers,
James.