Print Plot of String Equation in R

I am new to R and have this pretty simple question.

I am importing an equation from a json file called run.json. The JSON looks something like this.

    "equations": {
        "spec": {
            "expression": "x*x*x+2*x",
            "seed": 0,
            "range": 100
        }
    }

Im loading in the file like so

library("rjson")
result <- fromJSON(file = "run.json")

png('plot-spec.png', width=4, height=4, units='in', res=300)
specExpression = result$equations$spec$expression
seed = result$equations$spec$seed
range = result$equations$spec$range

# using noquote to remove quotes from the specExpression as it came in as a string
specEquation = function(x){noquote(specExpression)}
curve(
specEquation,
    from=seed,
    to=range,
    ylab="Y",
    xlab="X")

I keep getting the following error 'expr' did not evaluate to an object of length 'n'. Not too sure what to do with it