GlyphRun, problem with Tab Key rendering

Hello, I have a problem with rendering tab-key in GlyphRun.

Here is example:

ushort glyphIndex = glyphTypeface.CharacterToGlyphMap[temp[n]]; // Here I have error because when temp[n] is '\t' then this char is doesn't exist in this map(CMAP?)
GlyphIndexes.Add(glyphIndex);
double width = GlyphTypeface.AdvanceWidths[glyphIndex] * FontSize;
AdvanceWidths.Add(width);