Dissecting bits instead of bytes

I'm trying to get the first 4 bits and the last 4 bits of a byte.

proto_tree_add_bits_item(tree, hf_capability, tvb, bit_offset, 4, ENC_BIG_ENDIAN);
proto_tree_add_bits_item(tree, hf_capability, tvb, bit_offset+4, 4, ENC_BIG_ENDIAN);
{ &hf_capability,
    { "Capabilities", "status.capability",
    FT_UINT8, BASE_DEC, 
    VALS(capabilitynames), 0x0,
    NULL, HFILL }
},</code></pre><p>When I use this in wireshark, its clearly not happy.</p><pre><code>(lt-wireshark:18421): Pango-WARNING **: Invalid UTF-8 string passed to pango_layout_set_text()</code></pre><p>I think it's because its not a full 8 bits. Do I need to use a bitmask? How do I use a bitmask? I'm stumbling over syntax a lot, so please show me code examples of what I should do.</p><p>Here's where I'm mapping the integers to strings:</p><pre><code>static const value_string capabilitynames[] = {
{ 1, &quot;Switchable&quot; },
{ 2, &quot;Human&quot; },
{ 4, &quot;Mobile&quot; },

};