Dissolve after Union gives ERROR 999999

[BF_TadejBevk]

Hi,

I've got a polygon shapefile created by Union of two other shapefiles. After trying to Dissolve the Union shapefile across all fields (except OIDs) as suggested here, error 999999 is thrown with "Failed to execute (Dissolve)." 

The original shapefiles both have the same Field names, so after doing a Union one set of field names has "_1" appended. The dissolve I am actually trying to do is across six fields (let's call them Field1, Field2, Field3 and Field1_1, Field2_1, Field3_1). This gives the above error. If I however try to dissovle only across first set of fields, the Dissolve works. The same with the second set (with "_1" appended). It even works with any arbitrary selecton of up to 4 fields of interest - when the fifth is added I get the error.

All of the fields are type Text. Using ArcMap ArcGIS Desktop 10.7.1 on Windows 10 machine. 

I will be grateful for any idea and if more clarification is needed please ask.

Best to all!

[JoeBorgione]

Just a WAG (wild a$$ guess) but if your data is in a file geodatabase, try running check geometry, followed by a repair geometry as needed.

[BF_TadejBevk]

Thank you for the suggestion. I did this and no problems were reported. 

[DanPatterson]

Make of the Union a geodatase featureclass first.

Use that result as the input into the Dissolve.

Report if it works.

The shapefile as inputs and as outputs miss a bunch of geometry checks that are performed when gdb featureclasses aren't used.

If you need a shapefile output, then do it at the very end