diff --git a/src/hb-ot-shaper-thai.cc b/src/hb-ot-shaper-thai.cc
index e8410d2c3..a2081fd0f 100644
--- a/src/hb-ot-shaper-thai.cc
+++ b/src/hb-ot-shaper-thai.cc
@@ -279,7 +279,7 @@ preprocess_text_thai (const hb_ot_shape_plan_t *plan,
    * to be what Uniscribe and other engines implement.  According to Eric Muller:
    *
    * When you have a SARA AM, decompose it in NIKHAHIT + SARA AA, *and* move the
-   * NIKHAHIT backwards over any tone mark (0E48-0E4B).
+   * NIKHAHIT backwards over any above-base marks (0E31, 0E34-0E37, 0E47-0E4E).
    *
    * <0E14, 0E4B, 0E33> -> <0E14, 0E4D, 0E4B, 0E32>
    *
@@ -319,7 +319,7 @@ preprocess_text_thai (const hb_ot_shape_plan_t *plan,
 #define IS_SARA_AM(x) (((x) & ~0x0080u) == 0x0E33u)
 #define NIKHAHIT_FROM_SARA_AM(x) ((x) - 0x0E33u + 0x0E4Du)
 #define SARA_AA_FROM_SARA_AM(x) ((x) - 1)
-#define IS_TONE_MARK(x) (hb_in_ranges<hb_codepoint_t> ((x) & ~0x0080u, 0x0E34u, 0x0E37u, 0x0E47u, 0x0E4Eu, 0x0E31u, 0x0E31u))
+#define IS_ABOVE_BASE_MARK(x) (hb_in_ranges<hb_codepoint_t> ((x) & ~0x0080u, 0x0E34u, 0x0E37u, 0x0E47u, 0x0E4Eu, 0x0E31u, 0x0E31u))
 
   buffer->clear_output ();
   unsigned int count = buffer->len;
@@ -343,7 +343,7 @@ preprocess_text_thai (const hb_ot_shape_plan_t *plan,
 
     /* Ok, let's see... */
     unsigned int start = end - 2;
-    while (start > 0 && IS_TONE_MARK (buffer->out_info[start - 1].codepoint))
+    while (start > 0 && IS_ABOVE_BASE_MARK (buffer->out_info[start - 1].codepoint))
       start--;
 
     if (start + 2 < end)