Weird Dash in SSRS

I have an odd issue in SSRS. there's a weird Dash appearing in a textbox:-
Attachment 139873

I have no idea where this is coming from.

Here's the xml for that textbox:-

xml Code:

<Textbox Name="Textbox96">
        <CanGrow>true</CanGrow>
        <KeepTogether>true</KeepTogether>
        <Paragraphs>
          <Paragraph>
            <TextRuns>
              <TextRun>
                <Value>=First(Fields!ID6C.Value, "dsPerson")</Value>
                <ActionInfo>
                  <Actions>
                    <Action>
                      <Hyperlink>="javascript:void(window.open('MyServer?/BCC Reporting/Troubled Families/ThinkFamiliesReporting/Person Indicator Detail&amp;PersonID=" + cstr(Parameters!PersonID.Value) + "&amp;IndicatorCode=6C&amp;rc:Parameters=false','_blank'))"</Hyperlink>
                    </Action>
                  </Actions>
                </ActionInfo>
                <Style>
                  <FontSize>7pt</FontSize>
                  <Color>=iif(isNothing(First(Fields!ID6C.Value, "dsPerson")), "Black", "White")</Color>
                </Style>
              </TextRun>
            </TextRuns>
            <Style />
          </Paragraph>
        </Paragraphs>
        <rd:DefaultName>Textbox12</rd:DefaultName>
        <ActionInfo>
          <Actions>
            <Action>
              <Hyperlink>="javascript:void(window.open('http://MyServer?/BCC Reporting/Troubled Families/ThinkFamiliesReporting/Person Indicator Detail&amp;PersonID=" + cstr(Parameters!PersonID.Value) + "&amp;IndicatorCode=6C&amp;rc:Parameters=false','_blank'))"</Hyperlink>
            </Action>
          </Actions>
        </ActionInfo>
        <Top>11.01518cm</Top>
        <Left>30.35355cm</Left>
        <Height>0.4cm</Height>
        <Width>2.27839cm</Width>
        <ZIndex>88</ZIndex>
        <ToolTip>=First(Fields!ID6C.Value, "dsIndicatorValueDescriptions")</ToolTip>
        <Style>
          <Border>
            <Style>Solid</Style>
          </Border>
          <BackgroundColor>=iif(isNothing(First(Fields!ID6C.Value, "dsPerson")), "White", "Green")</BackgroundColor>
          <PaddingLeft>2pt</PaddingLeft>
          <PaddingRight>2pt</PaddingRight>
          <PaddingTop>2pt</PaddingTop>
          <PaddingBottom>2pt</PaddingBottom>
        </Style>
      </Textbox>

The value being returned from the dataset is NULL.

When I hover the mouse over the dash the cursor turns to a text selection cursor, implying that the dash is "real" text but I can't actually select it. I've even removed the text box and then put a new one on the report - the dash is still there.

Anyone got any suggestions as to where this might be coming from? I've looked at everything I can think of and I'm completely stumped.

Edit> A couple of other clues: It doesn't appear when I preview the report in visual studio. It does appear once deployed in IE, Chrome and Firefox. Deleting the deployed report and redeploying doesn't get rid of it.