Can't compute cross-correlation of two DataArray vectors?

[jiahao]

In[16]: using DataFrames, RDatasets
iris = dataset("datasets", "iris")
xcorr(iris[:SepalWidth], iris[:SepalLength])

Out[16]: `conv` has no method matching conv(::DataArray{Float64,1}, ::DataArray{Float64,1})
while loading In[16], in expression starting on line 1
 in xcorr at dsp.jl:160