diff --git a/src/hb-ot-shape-complex-arabic.cc b/src/hb-ot-shape-complex-arabic.cc
index 7c1b7bcbf..77a9c8230 100644
--- a/src/hb-ot-shape-complex-arabic.cc
+++ b/src/hb-ot-shape-complex-arabic.cc
@@ -67,6 +67,14 @@ static unsigned int get_joining_type (hb_codepoint_t u, hb_category_t gen_cat)
       return j_type;
   }
 
+  /* Mongolian joining data is not in ArabicJoining.txt yet */
+  if (unlikely (0x1800 <= u && u <= 0x18AF))
+  {
+    /* All letters, SIBE SYLLABLE BOUNDARY MARKER, and NIRUGU are D */
+    if (gen_cat == HB_CATEGORY_OTHER_LETTER || u == 0x1807 || u == 0x180A)
+      return JOINING_TYPE_D;
+  }
+
   if (unlikely ((u & ~(0x200C^0x200D)) == 0x200C)) {
     return u == 0x200C ? JOINING_TYPE_U : JOINING_TYPE_C;
   }
diff --git a/src/hb-ot-shape-complex-private.hh b/src/hb-ot-shape-complex-private.hh
index 788d18aa6..fed167dec 100644
--- a/src/hb-ot-shape-complex-private.hh
+++ b/src/hb-ot-shape-complex-private.hh
@@ -42,6 +42,7 @@ hb_ot_shape_complex_categorize (const hb_segment_properties_t *props)
     case HB_SCRIPT_NKO:
     case HB_SCRIPT_SYRIAC:
     case HB_SCRIPT_MANDAIC:
+    case HB_SCRIPT_MONGOLIAN:
       return hb_ot_complex_shaper_arabic;
 
     default: