IBM Directory Integrator error "unknown format type at"

So here's a strange one that I didn't find anywhere in my numerous searches, so I figured I would post it here. When running a TDI AssemblyLine, I got the following exception / error:

12:14:43,274 ERROR - [My-JDBC-Connector] CTGDIS810E handleException - cannot handle exception , addonly

java.lang.IllegalArgumentException: unknown format type at

        at com.ibm.icu.text.MessageFormat.makeFormat(MessageFormat.java:2086)

        at com.ibm.icu.text.MessageFormat.applyPattern(MessageFormat.java:589)

        at com.ibm.icu.text.MessageFormat.<init>(MessageFormat.java:437)

        at com.ibm.icu.text.MessageFormat.format(MessageFormat.java:1137)

        at com.ibm.di.util.ParameterSubstitution.substitute(ParameterSubstitution.java:229)

        at com.ibm.di.util.ParameterSubstitution.substitute(ParameterSubstitution.java:173)

        at com.ibm.di.util.ParameterSubstitution.substitute(ParameterSubstitution.java:245)

        at com.ibm.di.connector.JDBCConnector.putEntry(JDBCConnector.java:1202)

        at com.ibm.di.server.AssemblyLineComponent.executeOperation(AssemblyLineComponent.java:3351)

        at com.ibm.di.server.AssemblyLineComponent.add1(AssemblyLineComponent.java:2012)

        at com.ibm.di.server.AssemblyLineComponent.add(AssemblyLineComponent.java:1965)

        at com.ibm.di.server.AssemblyLine.msExecuteNextConnector(AssemblyLine.java:3780)

        at com.ibm.di.server.AssemblyLine.executeMainStep(AssemblyLine.java:3391)

        at com.ibm.di.server.AssemblyLine.executeMainLoop(AssemblyLine.java:2989)

        at com.ibm.di.server.AssemblyLine.executeMainLoop(AssemblyLine.java:2972)

        at com.ibm.di.server.AssemblyLine.executeAL(AssemblyLine.java:2939)

        at com.ibm.di.server.AssemblyLine.run(AssemblyLine.java:1317)

12:14:43,276 ERROR - CTGDIS266E Error in NextConnectorOperation. Exception occurred: java.lang.IllegalArgumentException: unknown format type at

java.lang.IllegalArgumentException: unknown format type at

        at com.ibm.icu.text.MessageFormat.makeFormat(MessageFormat.java:2086)

        at com.ibm.icu.text.MessageFormat.applyPattern(MessageFormat.java:589)

        at com.ibm.icu.text.MessageFormat.<init>(MessageFormat.java:437)

        at com.ibm.icu.text.MessageFormat.format(MessageFormat.java:1137)

        at com.ibm.di.util.ParameterSubstitution.substitute(ParameterSubstitution.java:229)

        at com.ibm.di.util.ParameterSubstitution.substitute(ParameterSubstitution.java:173)

        at com.ibm.di.util.ParameterSubstitution.substitute(ParameterSubstitution.java:245)

        at com.ibm.di.connector.JDBCConnector.putEntry(JDBCConnector.java:1202)

        at com.ibm.di.server.AssemblyLineComponent.executeOperation(AssemblyLineComponent.java:3351)

        at com.ibm.di.server.AssemblyLineComponent.add1(AssemblyLineComponent.java:2012)

        at com.ibm.di.server.AssemblyLineComponent.add(AssemblyLineComponent.java:1965)

        at com.ibm.di.server.AssemblyLine.msExecuteNextConnector(AssemblyLine.java:3780)

        at com.ibm.di.server.AssemblyLine.executeMainStep(AssemblyLine.java:3391)

        at com.ibm.di.server.AssemblyLine.executeMainLoop(AssemblyLine.java:2989)

        at com.ibm.di.server.AssemblyLine.executeMainLoop(AssemblyLine.java:2972)

        at com.ibm.di.server.AssemblyLine.executeAL(AssemblyLine.java:2939)

        at com.ibm.di.server.AssemblyLine.run(AssemblyLine.java:1317)

The problem was that I had a scriptable field in a connector that was set as "Text w/substitution", but needed to be set to "Advanced (JavaScript)". Sometimes (with only a tiny bit of JavaScript, for example), you can get away with this. But in my case, I had a LOT of JavaScript and this error was the result.

To figure that out, I put lots of debugging code in the JavaScript itself and in the Component Hooks around when this field should have been evaluated.