<h4 dir="auto">A small, complete example of the issue</h4>
<div class="highlight highlight-source-python notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="&gt;&gt;&gt; series = pd.Series({'a':3, 'b':7, 'c':np.inf, 'd':6})
&gt;&gt;&gt; series.argmax() # should be c
'b'
&gt;&gt;&gt; series.idxmax() # should be c
'b'
&gt;&gt;&gt; series.max() # this works properly
inf
&gt;&gt;&gt; series.index[np.argmax(np.array(series))] # this work-around works
'c'"><pre class="notranslate"><span class="pl-c1">&gt;&gt;</span><span class="pl-c1">&gt;</span> <span class="pl-s1">series</span> <span class="pl-c1">=</span> <span class="pl-s1">pd</span>.<span class="pl-v">Series</span>({<span class="pl-s">'a'</span>:<span class="pl-c1">3</span>, <span class="pl-s">'b'</span>:<span class="pl-c1">7</span>, <span class="pl-s">'c'</span>:<span class="pl-s1">np</span>.<span class="pl-s1">inf</span>, <span class="pl-s">'d'</span>:<span class="pl-c1">6</span>})
<span class="pl-c1">&gt;&gt;</span><span class="pl-c1">&gt;</span> <span class="pl-s1">series</span>.<span class="pl-en">argmax</span>() <span class="pl-c"># should be c</span>
<span class="pl-s">'b'</span>
<span class="pl-c1">&gt;&gt;</span><span class="pl-c1">&gt;</span> <span class="pl-s1">series</span>.<span class="pl-en">idxmax</span>() <span class="pl-c"># should be c</span>
<span class="pl-s">'b'</span>
<span class="pl-c1">&gt;&gt;</span><span class="pl-c1">&gt;</span> <span class="pl-s1">series</span>.<span class="pl-en">max</span>() <span class="pl-c"># this works properly</span>
<span class="pl-s1">inf</span>
<span class="pl-c1">&gt;&gt;</span><span class="pl-c1">&gt;</span> <span class="pl-s1">series</span>.<span class="pl-s1">index</span>[<span class="pl-s1">np</span>.<span class="pl-en">argmax</span>(<span class="pl-s1">np</span>.<span class="pl-en">array</span>(<span class="pl-s1">series</span>))] <span class="pl-c"># this work-around works</span>
<span class="pl-s">'c'</span></pre></div>
<h4 dir="auto">Expected Output</h4>
<div class="highlight highlight-source-python notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="&gt;&gt;&gt; series = pd.Series({'a':3, 'b':7, 'c':np.inf, 'd':6})
&gt;&gt;&gt; series.argmax()
'c'
&gt;&gt;&gt; series.idxmax()
'c'"><pre class="notranslate"><span class="pl-c1">&gt;&gt;</span><span class="pl-c1">&gt;</span> <span class="pl-s1">series</span> <span class="pl-c1">=</span> <span class="pl-s1">pd</span>.<span class="pl-v">Series</span>({<span class="pl-s">'a'</span>:<span class="pl-c1">3</span>, <span class="pl-s">'b'</span>:<span class="pl-c1">7</span>, <span class="pl-s">'c'</span>:<span class="pl-s1">np</span>.<span class="pl-s1">inf</span>, <span class="pl-s">'d'</span>:<span class="pl-c1">6</span>})
<span class="pl-c1">&gt;&gt;</span><span class="pl-c1">&gt;</span> <span class="pl-s1">series</span>.<span class="pl-en">argmax</span>()
<span class="pl-s">'c'</span>
<span class="pl-c1">&gt;&gt;</span><span class="pl-c1">&gt;</span> <span class="pl-s1">series</span>.<span class="pl-en">idxmax</span>()
<span class="pl-s">'c'</span></pre></div>
<h4 dir="auto">Output of <code class="notranslate">pd.show_versions()</code></h4>
<details>
# Paste the output here pd.show_versions() here
```python
&gt;&gt;&gt; pd.show_versions()
<h2 dir="auto">INSTALLED VERSIONS</h2>
<p dir="auto">commit: None<br>
python: 3.5.2.final.0<br>
python-bits: 64<br>
OS: Darwin<br>
OS-release: 15.6.0<br>
machine: x86_64<br>
processor: i386<br>
byteorder: little<br>
LC_ALL: None<br>
LANG: en_US.UTF-8<br>
LOCALE: en_US.UTF-8</p>
<p dir="auto">pandas: 0.19.0<br>
nose: 1.3.7<br>
pip: 8.1.2<br>
setuptools: 23.0.0<br>
Cython: 0.24<br>
numpy: 1.11.2<br>
scipy: 0.18.1<br>
statsmodels: 0.6.1<br>
xarray: None<br>
IPython: 4.2.0<br>
sphinx: 1.4.1<br>
patsy: 0.4.1<br>
dateutil: 2.5.3<br>
pytz: 2016.7<br>
blosc: None<br>
bottleneck: 1.1.0<br>
tables: 3.2.2<br>
numexpr: 2.6.0<br>
matplotlib: 1.5.1<br>
openpyxl: 2.3.2<br>
xlrd: 1.0.0<br>
xlwt: 1.1.2<br>
xlsxwriter: 0.9.2<br>
lxml: 3.6.0<br>
bs4: 4.4.1<br>
html5lib: None<br>
httplib2: 0.9.2<br>
apiclient: None<br>
sqlalchemy: 1.0.13<br>
pymysql: None<br>
psycopg2: 2.6.2 (dt dec pq3 ext lo64)<br>
jinja2: 2.8<br>
boto: 2.40.0<br>
pandas_datareader: None</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="&lt;/details&gt;"><pre class="notranslate"><code class="notranslate">&lt;/details&gt;
</code></pre></div></details>
<h4 dir="auto">Code Sample, a copy-pastable example if possible</h4>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="In [3]: s = pd.Series([1, 2, np.inf])

In [4]: s.argmax()
Out[4]: 1

In [5]: s.argmax(skipna=False)
Out[5]: nan
"><pre class="notranslate"><code class="notranslate">In [3]: s = pd.Series([1, 2, np.inf])

In [4]: s.argmax()
Out[4]: 1

In [5]: s.argmax(skipna=False)
Out[5]: nan

</code></pre></div>
<h4 dir="auto">Expected Output</h4>
<p dir="auto">2 in both cases.</p>
<h4 dir="auto">output of <code class="notranslate">pd.show_versions()</code></h4>
<p dir="auto">In [6]: pd.show_versions()</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="INSTALLED VERSIONS
------------------
commit: a63bd12529ff309d957d714825b1753d0e02b7fa
python: 3.5.1.final.0
python-bits: 64
OS: Linux
OS-release: 4.5.0-2-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: it_IT.utf8
LOCALE: it_IT.UTF-8

pandas: 0.18.1+174.ga63bd12
nose: 1.3.7
pip: 1.5.6
setuptools: 18.4
Cython: 0.23.4
numpy: 1.10.4
scipy: 0.16.0
statsmodels: 0.8.0.dev0+111ddc0
xarray: None
IPython: 5.0.0.dev
sphinx: 1.3.1
patsy: 0.3.0-dev
dateutil: 2.2
pytz: 2012c
blosc: None
bottleneck: 1.1.0dev
tables: 3.2.2
numexpr: 2.5
matplotlib: 1.5.1
openpyxl: None
xlrd: 0.9.4
xlwt: 1.1.2
xlsxwriter: 0.7.3
lxml: None
bs4: 4.4.0
html5lib: 0.999
httplib2: 0.9.1
apiclient: 1.5.0
sqlalchemy: 1.0.11
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.38.0
pandas_datareader: 0.2.1
"><pre class="notranslate"><code class="notranslate">INSTALLED VERSIONS
------------------
commit: a63bd12529ff309d957d714825b1753d0e02b7fa
python: 3.5.1.final.0
python-bits: 64
OS: Linux
OS-release: 4.5.0-2-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: it_IT.utf8
LOCALE: it_IT.UTF-8

pandas: 0.18.1+174.ga63bd12
nose: 1.3.7
pip: 1.5.6
setuptools: 18.4
Cython: 0.23.4
numpy: 1.10.4
scipy: 0.16.0
statsmodels: 0.8.0.dev0+111ddc0
xarray: None
IPython: 5.0.0.dev
sphinx: 1.3.1
patsy: 0.3.0-dev
dateutil: 2.2
pytz: 2012c
blosc: None
bottleneck: 1.1.0dev
tables: 3.2.2
numexpr: 2.5
matplotlib: 1.5.1
openpyxl: None
xlrd: 0.9.4
xlwt: 1.1.2
xlsxwriter: 0.7.3
lxml: None
bs4: 4.4.0
html5lib: 0.999
httplib2: 0.9.1
apiclient: 1.5.0
sqlalchemy: 1.0.11
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.38.0
pandas_datareader: 0.2.1

</code></pre></div>