diff --git a/src/harfbuzz-tibetan.c b/src/harfbuzz-tibetan.c
index bfa31b1..847ac52 100644
--- a/src/harfbuzz-tibetan.c
+++ b/src/harfbuzz-tibetan.c
@@ -90,7 +90,7 @@ static const unsigned char tibetanForm[0x80] = {
 
 
 #define tibetan_form(c) \
-    (TibetanForm)tibetanForm[c - 0x0f40]
+    ((c) >= 0x0f40 && (c) <= 0x0fc0 ? (TibetanForm)tibetanForm[(c) - 0x0f40] : TibetanOther)
 
 static const HB_OpenTypeFeature tibetan_features[] = {
     { HB_MAKE_TAG('c', 'c', 'm', 'p'), CcmpProperty },