diff --git a/src/hb-ot-shape-complex-arabic.cc b/src/hb-ot-shape-complex-arabic.cc
index ae9086412..fc8258111 100644
--- a/src/hb-ot-shape-complex-arabic.cc
+++ b/src/hb-ot-shape-complex-arabic.cc
@@ -61,7 +61,7 @@ static unsigned int get_joining_type (hb_codepoint_t u, hb_unicode_general_categ
   if (likely (j_type != JOINING_TYPE_X))
     return j_type;
 
-  return (FLAG(gen_cat) &
+  return (FLAG_SAFE(gen_cat) &
 	  (FLAG(HB_UNICODE_GENERAL_CATEGORY_NON_SPACING_MARK) |
 	   FLAG(HB_UNICODE_GENERAL_CATEGORY_ENCLOSING_MARK) |
 	   FLAG(HB_UNICODE_GENERAL_CATEGORY_FORMAT))
diff --git a/src/hb-ot-shape-complex-indic.cc b/src/hb-ot-shape-complex-indic.cc
index 772360041..1dc84be2e 100644
--- a/src/hb-ot-shape-complex-indic.cc
+++ b/src/hb-ot-shape-complex-indic.cc
@@ -142,7 +142,7 @@ is_one_of (const hb_glyph_info_t &info, unsigned int flags)
 {
   /* If it ligated, all bets are off. */
   if (_hb_glyph_info_ligated (&info)) return false;
-  return !!(FLAG (info.indic_category()) & flags);
+  return !!(FLAG_SAFE (info.indic_category()) & flags);
 }
 
 static inline bool
@@ -237,7 +237,7 @@ set_indic_properties (hb_glyph_info_t &info)
    * Re-assign position.
    */
 
-  if ((FLAG (cat) & CONSONANT_FLAGS))
+  if ((FLAG_SAFE (cat) & CONSONANT_FLAGS))
   {
     pos = POS_BASE_C;
     if (is_ra (u))
@@ -247,7 +247,7 @@ set_indic_properties (hb_glyph_info_t &info)
   {
     pos = matra_position (u, pos);
   }
-  else if ((FLAG (cat) & (FLAG (OT_SM) | FLAG (OT_VD) | FLAG (OT_A) | FLAG (OT_Symbol))))
+  else if ((FLAG_SAFE (cat) & (FLAG (OT_SM) | FLAG (OT_VD) | FLAG (OT_A) | FLAG (OT_Symbol))))
   {
     pos = POS_SMVD;
   }
@@ -963,7 +963,7 @@ initial_reordering_consonant_syllable (const hb_ot_shape_plan_t *plan,
     indic_position_t last_pos = POS_START;
     for (unsigned int i = start; i < end; i++)
     {
-      if ((FLAG (info[i].indic_category()) & (JOINER_FLAGS | FLAG (OT_N) | FLAG (OT_RS) | MEDIAL_FLAGS | HALANT_OR_COENG_FLAGS)))
+      if ((FLAG_SAFE (info[i].indic_category()) & (JOINER_FLAGS | FLAG (OT_N) | FLAG (OT_RS) | MEDIAL_FLAGS | HALANT_OR_COENG_FLAGS)))
       {
 	info[i].indic_position() = last_pos;
 	if (unlikely (info[i].indic_category() == OT_H &&
@@ -1550,7 +1550,7 @@ final_reordering_syllable (const hb_ot_shape_plan_t *plan,
     {
       new_reph_pos = base;
       while (new_reph_pos < end &&
-	     !( FLAG (info[new_reph_pos + 1].indic_position()) & (FLAG (POS_POST_C) | FLAG (POS_AFTER_POST) | FLAG (POS_SMVD))))
+	     !( FLAG_SAFE (info[new_reph_pos + 1].indic_position()) & (FLAG (POS_POST_C) | FLAG (POS_AFTER_POST) | FLAG (POS_SMVD))))
 	new_reph_pos++;
       if (new_reph_pos < end)
         goto reph_move;
@@ -1701,7 +1701,7 @@ final_reordering_syllable (const hb_ot_shape_plan_t *plan,
   /* Apply 'init' to the Left Matra if it's a word start. */
   if (info[start].indic_position () == POS_PRE_M &&
       (!start ||
-       !(FLAG (_hb_glyph_info_get_general_category (&info[start - 1])) &
+       !(FLAG_SAFE (_hb_glyph_info_get_general_category (&info[start - 1])) &
 	 FLAG_RANGE (HB_UNICODE_GENERAL_CATEGORY_FORMAT, HB_UNICODE_GENERAL_CATEGORY_NON_SPACING_MARK))))
     info[start].mask |= indic_plan->mask_array[INIT];
 
diff --git a/src/hb-ot-shape-complex-myanmar.cc b/src/hb-ot-shape-complex-myanmar.cc
index d016380cc..29c1ae827 100644
--- a/src/hb-ot-shape-complex-myanmar.cc
+++ b/src/hb-ot-shape-complex-myanmar.cc
@@ -154,7 +154,7 @@ is_one_of (const hb_glyph_info_t &info, unsigned int flags)
 {
   /* If it ligated, all bets are off. */
   if (_hb_glyph_info_ligated (&info)) return false;
-  return !!(FLAG (info.myanmar_category()) & flags);
+  return !!(FLAG_SAFE (info.myanmar_category()) & flags);
 }
 
 static inline bool
diff --git a/src/hb-private.hh b/src/hb-private.hh
index e941126aa..dc67b3a49 100644
--- a/src/hb-private.hh
+++ b/src/hb-private.hh
@@ -846,9 +846,10 @@ hb_in_ranges (T u, T lo1, T hi1, T lo2, T hi2, T lo3, T hi3)
 
 /* Useful for set-operations on small enums.
  * For example, for testing "x ∈ {x1, x2, x3}" use:
- * (FLAG(x) & (FLAG(x1) | FLAG(x2) | FLAG(x3)))
+ * (FLAG_SAFE(x) & (FLAG(x1) | FLAG(x2) | FLAG(x3)))
  */
-#define FLAG(x) (1U<<(x))
+#define FLAG(x) (ASSERT_STATIC_EXPR_ZERO ((x) < 32) + (1U << (x)))
+#define FLAG_SAFE(x) (1U << (x))
 #define FLAG_RANGE(x,y) (ASSERT_STATIC_EXPR_ZERO ((x) < (y)) + FLAG(y+1) - FLAG(x))
 
 
diff --git a/src/hb-unicode-private.hh b/src/hb-unicode-private.hh
index a2c59da27..968bca556 100644
--- a/src/hb-unicode-private.hh
+++ b/src/hb-unicode-private.hh
@@ -308,7 +308,7 @@ extern HB_INTERNAL const hb_unicode_funcs_t _hb_unicode_funcs_nil;
 /* Misc */
 
 #define HB_UNICODE_GENERAL_CATEGORY_IS_MARK(gen_cat) \
-	(FLAG (gen_cat) & \
+	(FLAG_SAFE (gen_cat) & \
 	 (FLAG (HB_UNICODE_GENERAL_CATEGORY_SPACING_MARK) | \
 	  FLAG (HB_UNICODE_GENERAL_CATEGORY_ENCLOSING_MARK) | \
 	  FLAG (HB_UNICODE_GENERAL_CATEGORY_NON_SPACING_MARK)))