title: Errors using CausalImpact package with Zoo objects
body: <p>I'm trying to model the impact of storms on sales patterns using the CausalImpact package. When I create a zoo object and pass it to the model I receive an error. I've read through the documentation and can't figure out what I'm doing differently from the examples in the documentation.</p>
  
  <p>I'm working with the following data.frame:</p>
  
  <pre><code>&gt; head(my.data)
          date    sales    units
  1 2014-10-17 71319.85 21436.64
  2 2014-10-18 88598.26 26755.79
  3 2014-10-19 95768.29 29823.86
  4 2014-10-20 62303.04 19417.71
  5 2014-10-21 56477.57 17562.21
  6 2014-10-22 54890.39 16946.43
  </code></pre>
  
  <p>Then I'm converting it to a zoo object: </p>
  
  <pre><code>my.data&lt;- zoo( my.data[ ,c('sales','units')], my.data[,'date'] )
  
  &gt; str(my.data)
  ‘zoo’ series from 2014-10-17 to 2017-04-13
    Data: num [1:907, 1:2] 71320 88598 95768 62303 56478 ...
   - attr(*, "dimnames")=List of 2
    ..$ : NULL
    ..$ : chr [1:2] "sales" "units"
    Index:  Date[1:907], format: "2014-10-17" "2014-10-18" "2014-10-19" ...
  </code></pre>
  
  <p>Then I set the pre and post periods and run the model:</p>
  
  <pre><code>pre.period &lt;- as.Date(c('2015-10-17','2017-03-09'))
  post.period &lt;- as.Date(c('2017-03-10','2017-04-13'))
  
  library(CausalImpact)
  impact&lt;- CausalImpact(data = my.data, pre.period = pre.period, post.period = post.period, alpha = .01)
  </code></pre>
  
  <p>But I'm receiving this error: </p>
  
  <pre><code>&gt; impact&lt;- CausalImpact(data = my.data, pre.period = pre.period, post.period = post.period, alpha = .05)
  Error in bsts(formula, data = data, state.specification = ss, expected.model.size = kStaticRegressionExpectedModelSize,  : 
    Caught exception with the following error message: 
  BregVsSampler did not start with a legal configuration.
  Selector vector:  11
  beta: 0 0
  </code></pre>
  
  <p>I've used this package successfully with univariate time series data, but cant identify why this isn't working. </p>
  
  <p>Thank you for your help! </p>