diff --git a/src/autofit/afadjust.c b/src/autofit/afadjust.c
index 6637cacfc..a1aa45914 100644
--- a/src/autofit/afadjust.c
+++ b/src/autofit/afadjust.c
@@ -1405,7 +1405,8 @@
         FT_TRACE4(( "  " ));
         idx = HB_SET_VALUE_INVALID;
         while ( hb( set_next )( gsub_lookups, &idx ) )
-          if ( globals->gsub_lookups_single_alternate[idx] )
+          if ( idx < globals->gsub_lookup_count            &&
+               globals->gsub_lookups_single_alternate[idx] )
           {
             have_idx = TRUE;
             FT_TRACE4(( "  %u", idx ));
@@ -1428,9 +1429,17 @@
       idx = HB_SET_VALUE_INVALID;
       while ( hb( set_next )( gsub_lookups, &idx ) )
       {
-        FT_UInt32  offset = globals->gsub_lookups_single_alternate[idx];
+        FT_UInt32  offset;
 
 
+        /* HarfBuzz only validates lookup indices while   */
+        /* processing lookups, not while collecting them, */
+        /* so we have to do that by ourselves.            */
+        if ( idx < globals->gsub_lookup_count )
+          offset = globals->gsub_lookups_single_alternate[idx];
+        else
+          offset = 0;
+
         /* Put all substitutions into a single hash table.  Note that   */
         /* the hash values usually contain more than a single character */
         /* code; this can happen if different 'SingleSubst' subtables   */
diff --git a/src/autofit/afglobal.h b/src/autofit/afglobal.h
index 362f56e29..dc0611594 100644
--- a/src/autofit/afglobal.h
+++ b/src/autofit/afglobal.h
@@ -116,7 +116,9 @@ FT_BEGIN_HEADER
 
     /* The GSUB table. */
     FT_Byte*         gsub;
-    /* Lookup offsets, with only SingleSubst and AlternateSubst non-NULL. */
+    /* An array of lookup offsets (of `gsub_lookup_count` elements), */
+    /* with only SingleSubst and AlternateSubst lookups non-NULL.    */
+    FT_UShort        gsub_lookup_count;
     FT_UInt32*       gsub_lookups_single_alternate;
 #endif
 
diff --git a/src/autofit/afgsub.c b/src/autofit/afgsub.c
index f458559d0..a9e3996a7 100644
--- a/src/autofit/afgsub.c
+++ b/src/autofit/afgsub.c
@@ -364,6 +364,7 @@
     }
 
     globals->gsub                          = gsub;
+    globals->gsub_lookup_count             = lookupCount;
     globals->gsub_lookups_single_alternate = gsub_lookups_single_alternate;
 
     return;