How to disambiguate these two grammar rules

[ballcoach12]

I have two grammar that is intended to support JSON-like syntax. It has two grammar rules that the parser finds to be ambiguous. The first rule is intended to specify a field name and value:

IsTerm:
   not?='!' columnName=ID ':' value=ColumnValue 
;


ColumnValue:
    StringExpression | ID | BooleanExpression | NumberExpression
    ;

The second rule is intended to specify a column name and its data type:

FieldMember:
    name=ID ':' type=TypeReference;

TypeReference: reference=[StructDeclaration:ID] 
    | primitive=("string" | "integer" | "double" | "boolean" | "void") ;

The parser seems to always match the FieldMember rule where I want it to match the IsTerm rule.

How should I go about disambiguating these two rules?

Thanks

[ballcoach12]

It turns out that it's the guard condition not?='!' of the IsTerm rule that was throwing it off. Changing the rule to :

IsTerm:
  not?='!'? column=ID ':' value=ColumnValue 
;

fixed the problem.