Media ScenarioEngine.exe returned exit code: 0x84B40001 upon silent install of SQL Server 2014

I'm trying to run a silent install of SQL Server 2014 and getting the error: Media ScenarioEngine.exe returned exit code: 0x84B40001. I call this install from a C# program but if I run the same command from a command line it works fine. Here's the command I'm issuing:

SQLExpr_x64_ENU.exe /SkipInstallerRunCheck /QUIETSIMPLE /SAPWD="OurPWHere" /INSTANCENAME="PACSGEAR" /INSTANCEID="PACSGEAR" /ACTION=Install /FEATURES=SQLENGINE /AGTSVCSTARTUPTYPE=Manual /ISSVCSTARTUPTYPE=Automatic /ISSVCACCOUNT="NT AUTHORITY\NetworkService" /ASSVCSTARTUPTYPE=Automatic /SQLSVCSTARTUPTYPE=Automatic /SQLSVCACCOUNT="NT AUTHORITY\NETWORK SERVICE" /SQLSYSADMINACCOUNTS="BUILTIN\Administrators" /SECURITYMODE=SQL /IACCEPTSQLSERVERLICENSETERMS /SKIPRULES=RebootRequiredCheck

And here's a snippet of the c# code that fails

string commandLine = "/SkipInstallerRunCheck /QS /SAPWD=" + sSQLOurSAPwd + " " +
                "/INSTANCENAME=" + sqlInstance + " /INSTANCEID=" + sqlInstance + " /ACTION=Install " +
                "/FEATURES=SQLENGINE /AGTSVCSTARTUPTYPE=Manual /ISSVCSTARTUPTYPE=Automatic " +
                "/ISSVCACCOUNT=\"NT AUTHORITY\\NetworkService\" /ASSVCSTARTUPTYPE=Automatic /SQLSVCSTARTUPTYPE=Automatic " +
                "/SQLSVCACCOUNT=\"NT AUTHORITY\\NETWORK SERVICE\" /SQLSYSADMINACCOUNTS=\"BUILTIN\\Administrators\" /SECURITYMODE=SQL " +
                "/IACCEPTSQLSERVERLICENSETERMS /SKIPRULES=RebootRequiredCheck";

Process installerProcess = Process.Start(sSQLSetup, commandLine);

I've googled the exact error message and found nothing. Please help!