value error in python statsmodels.tsa.seasonal : <p>I have this dataframe with date time indices:</p>

<pre><code>ts_log:
</code></pre>

<p><code>date   price_per_unit
2013-04-04  12.762369
2013-04-05  12.777120
2013-04-06  12.773146
2013-04-07  12.780774
2013-04-08  12.786835</code></p>

<p>I have this piece of code for <code>decomposition</code> 
`</p>

<pre><code>from statsmodels.tsa.seasonal import seasonal_decompose
decomposition = seasonal_decompose(ts_log)

trend = decomposition.trend
seasonal = decomposition.seasonal
residual = decomposition.resid
</code></pre>

<p>but in the line <code>decomposition = seasonal_decompose(ts_log)</code>
i got this error :</p>

<pre><code>ValueError: You must specify a freq or x must be a pandas object with a timeseries index
</code></pre>

<p>Where is the problem?</p>
