D3 – Issue Switching X Axis on Line Graph From Date to Month

I’m currently working with a csv and am importing it so that I have three columns: DateTime, Month, and Count.

If I create a line graph using DateTime and Count everything works just fine. When I switch to using Month and Count, however, I get the error seen above.

I’ve found some other posts that are similar and tried a few different options such as switching the x scale from scaleTime() to scaleOrdinal(). I also tried adding the tickformat to my xaxis like so:

var xAxis_a = d3.axisBottom(xScale)
    .tickFormat(d3.time.format("%b-%y"))
    .ticks(15);

Unfortunately, none of these changes have worked. So, how can I get a line plot with my x-axis displaying an abbreviated month and year?