"Variable context not set" error with mutate_at, dplyr version >= 0.7 : <p>This code used to work, as of about May 1 2017 (<code>dplyr</code> version 0.5.0). With <code>dplyr</code> version 0.7 it fails with <code>Error: Variable context not set</code>.  I couldn't find the solution googling around or looking in the <a href="https://github.com/tidyverse/dplyr/blob/master/NEWS.md" rel="noreferrer">dplyr NEWS file</a>.</p>

<p>This part is fine (setting up the example - could probably be simplified ...)</p>

<pre><code>xx &lt;- data.frame(stud_number=1:3,HW1=rep(0,3),HW2=c(NA,1,1),junk=rep(NA,3))
repl_1_NA &lt;- function(x) { return(replace(x,which(x==1),NA)) }
hw1 &lt;- xx %&gt;% select(c(stud_number,starts_with("HW")))
</code></pre>

<p>Now try to use <code>mutate_at()</code>: fails with <code>dplyr</code> version >= 0.7.0</p>

<pre><code>hw1 %&gt;% mutate_at(starts_with("HW"),repl_1_NA)
</code></pre>
