prerequisiteResearchSubject != 0

C:\Projects\P4\SinsRebellion\Release\CodeSource\GS\Window\Screens\ResearchScreen.cpp(874): assert! [prerequisiteResearchSubject != 0]

I get this error when I add a prerequisite to one of my custom research items, but I can't see anything wrong with the file or its references. Without the prerequisite I get no errors.

Basically I've added another level of shield upgrades for the Advent.

TXT
entityType "ResearchSubject"
hudIcon "HUDICON_RESEARCHSUBJECT_SHIELDMASTERY"
smallHudIcon "HUDICON_RESEARCHSUBJECT_SHIELDMASTERY"
infoCardIcon ""
NameStringID "IDS_RESEARCHSUBJECT_SHIELDMASTERY_NAME"
DescriptionStringID "IDS_RESEARCHSUBJECT_SHIELDMASTERY_DESCRIPTION"
researchWindowLocation
    block 0
    pos [ 6 , 4 ]
ResearchField "Combat"
Prerequisites
    NumResearchPrerequisites 1
    ResearchPrerequisite
        Subject "RESEARCHSUBJECT_SHIELDMAXMITIGATION"
        Level 2
    RequiredFactionNameID ""
    RequiredCompletedResearchSubjects 0
MinimumArtifactLevel -1
BaseUpgradeTime 105.000000
PerLevelUpgradeTime 10.000000
BaseCost
    credits 1600.000000
    metal 300.000000
    crystal 475.000000
PerLevelCostIncrease
    credits 100.000000
    metal 25.000000
    crystal 25.000000
Tier 6
onlyWorksIfTierLabsExist FALSE
MaxNumResearchLevels 2
priority 1.000000
researchBoolModifiers 0
researchFloatModifiers 2
researchModifier
    modifierType "ShieldPointsMaxAdjustment"
    baseValue 0.000000
    perLevelValue 0.050000
researchModifier
    modifierType "ShieldPointsRegenAdjustment"
    baseValue 0.000000
    perLevelValue 0.050000
artifactPicture ""
uniqueOverlayBrush "RebellionResearchOverlay"
dlcId 204880