# 6.7.2. Missing expression error

When setting up your problem, you may encounter a missing expression error when you run Aria. This will typically start with:

    Missing expression FOO on subdomain [Etet_Te4_g4/Etet_Te4_g4/Etet_Te4_g4 on block_3/Etet_Te4_g4_default_integration_rule/-1]

An expression in Aria is another name for a defined property in the expression graph (like `DENSITY` or `SPECIFIC_HEAT`). If you see an error about a missing expression `FOO`, it means that some model you are using has listed `FOO` as a prerequisite, but you have not defined a model for `FOO`. The error message will also tell you which mesh block `FOO` is missing on (`block_3` in the example above) - so the solution is usually to add a definition for `FOO` in the material used on that block.

For example, if you had defined `DENSITY` using some model that uses `FOO` as an input but did not define `FOO` you would receive this error.

    Begin Aria material blah
        # this fictitous model calculates density = 10 + foo
        Density = Computed_From_Foo

        # If we don't define "foo" we will get a missing expression error
        # Foo = constant value = 5
    End

If this error occurs on a block subdomain tag, the property is likely missing in the associated block’s material property definition. For surface subdomains (e.g. surface BCs, interface BCs, etc) the expression can be defined either in the surface material (if defined) or the owning block’s material. For contact enforcements, properties can still be defined in either the surface of block materials, but can also be provided as contact material properties.