RSelenium error - com.google.common.collect.Maps$TransformedEntriesMap incompatible with java.lang.CharSequence

I am trying to use RSelenium automate a data entry task for my job in which I must input some variables into an online system. There are 5 variables to enter, and my code works for all of them except for one, the "First Name" entry.

remDr$findElement("css selector", "#ctl00_CPHMain_frvProfile_txtFederatedUserID")$sendKeysToElement(list(NetID[i]))

remDr$findElement("css selector", "#ctl00_CPHMain_frvProfile_txtUserName")$sendKeysToElement(list(ID[i]))

remDr$findElement("css selector", "#ctl00_CPHMain_frvProfile_txtFirstName")$sendKeysToElement(list(First[i]))

remDr$findElement("css selector", "#ctl00_CPHMain_frvProfile_txtLastName")$sendKeysToElement(list(Last[i]))

remDr$findElement("css selector", "#ctl00_CPHMain_frvProfile_txtEmail")$sendKeysToElement(list(Email[i]))

The top two and bottom two both enter the data perfectly, but the middle snippet throws:

Selenium message:com.google.common.collect.Maps$TransformedEntriesMap incompatible with java.lang.CharSequence

Error:   Summary: UnknownError
     Detail: An unknown server-side error occurred while processing the command.
     Further Details: run errorDetails method

In my Google searching and troubleshooting I have never seen this error, and it's very strange to me that it only applies to one of my elements. Anyone able to help?

Edit: Here is the HTML of the page I am pulling from:

<a href="javascript:WebForm_DoPostBackWithOptions(new WebForm_PostBackOptions(&quot;ctl00$CPHMain$frvProfile$ctl02&quot;, &quot;&quot;, true, &quot;&quot;, &quot;&quot;, false, true))">Save & Close</a>
                </li>
            </ul>
        </div>
        <table align="left" style="width: 100%">

            <tr>
        <td align="right">
                    <strong>
                        <label for="ctl00_CPHMain_frvProfile_txtFederatedUserID" id="ctl00_CPHMain_frvProfile_lblFederatedUserID">Federated User ID</label>
                    </strong>
                </td>
        <td colspan="2">
                    <input name="ctl00$CPHMain$frvProfile$txtFederatedUserID" type="text" id="ctl00_CPHMain_frvProfile_txtFederatedUserID" />

                    <span id="ctl00_CPHMain_frvProfile_rfvFederatedUserID" style="display:none;">*</span>

                    <span id="ctl00_CPHMain_frvProfile_cvFedID" style="display:none;"><br/>*Federated User ID already in use</span>
                </td>
    </tr>

            <tr>
        <td>
                    &nbsp;
                </td>
        <td>
                    &nbsp;
                </td>
    </tr>

            <tr>
                <td align="right">
                    <label for="ctl00_CPHMain_frvProfile_txtUserName" id="ctl00_CPHMain_frvProfile_lbltxtUserName">Username</label>
                </td>
                <td colspan="2">
                    <input name="ctl00$CPHMain$frvProfile$txtUserName" type="text" id="ctl00_CPHMain_frvProfile_txtUserName" />
                    <span id="ctl00_CPHMain_frvProfile_rfvUserName" style="display:none;">*</span>
                    <span id="ctl00_CPHMain_frvProfile_cvUserName" style="display:none;"><br/>*Username already in use</span>
                </td>
            </tr>
            <tr>
                <td>
                    &nbsp;
                </td>
                <td>
                    &nbsp;
                </td>
            </tr>
                <td align="right">
                    <label for="ctl00_CPHMain_frvProfile_txtFirstName" id="ctl00_CPHMain_frvProfile_lbltxtFirstName">First Name</label>
                </td>
                <td colspan="2">
                    <input name="ctl00$CPHMain$frvProfile$txtFirstName" type="text" id="ctl00_CPHMain_frvProfile_txtFirstName" />
                    <span id="ctl00_CPHMain_frvProfile_rfvFirstName" style="display:none;">*</span>
                </td>
            </tr>