diff --git a/stb_truetype.h b/stb_truetype.h
index 8dd9690..0f18b6b 100644
--- a/stb_truetype.h
+++ b/stb_truetype.h
@@ -735,6 +735,7 @@ STBTT_DEF int stbtt_FindGlyphIndex(const stbtt_fontinfo *info, int unicode_codep
 // and you want a speed-up, call this function with the character you're
 // going to process, then use glyph-based functions instead of the
 // codepoint-based functions.
+// Returns 0 if the character codepoint is not defined in the font.
 
 
 //////////////////////////////////////////////////////////////////////////////