diff --git a/Lib/fontTools/feaLib/builder.py b/Lib/fontTools/feaLib/builder.py
index b26426298..a9ecd9d9e 100644
--- a/Lib/fontTools/feaLib/builder.py
+++ b/Lib/fontTools/feaLib/builder.py
@@ -898,9 +898,7 @@ class AlternateSubstBuilder(LookupBuilder):
                 self.alternates == other.alternates)
 
     def build(self):
-        subtable = otTables.AlternateSubst()
-        subtable.Format = 1
-        subtable.alternates = self.alternates
+        subtable = otlBuilder.buildAlternateSubst(self.alternates)
         return self.buildLookup_([subtable])
 
     def getAlternateGlyphs(self):
diff --git a/Lib/fontTools/feaLib/testdata/GSUB_3.ttx b/Lib/fontTools/feaLib/testdata/GSUB_3.ttx
index 0a69377ab..cb08e90ca 100644
--- a/Lib/fontTools/feaLib/testdata/GSUB_3.ttx
+++ b/Lib/fontTools/feaLib/testdata/GSUB_3.ttx
@@ -41,7 +41,7 @@
         <!-- LookupType=3 -->
         <LookupFlag value="0"/>
         <!-- SubTableCount=1 -->
-        <AlternateSubst index="0" Format="1">
+        <AlternateSubst index="0">
           <AlternateSet glyph="A">
             <Alternate glyph="A.alt1"/>
             <Alternate glyph="A.alt2"/>
diff --git a/Lib/fontTools/feaLib/testdata/GSUB_6.ttx b/Lib/fontTools/feaLib/testdata/GSUB_6.ttx
index 764c9d1ad..fcc3ceb0d 100644
--- a/Lib/fontTools/feaLib/testdata/GSUB_6.ttx
+++ b/Lib/fontTools/feaLib/testdata/GSUB_6.ttx
@@ -158,7 +158,7 @@
         <!-- LookupType=3 -->
         <LookupFlag value="0"/>
         <!-- SubTableCount=1 -->
-        <AlternateSubst index="0" Format="1">
+        <AlternateSubst index="0">
           <AlternateSet glyph="e">
             <Alternate glyph="e"/>
             <Alternate glyph="e.begin"/>
diff --git a/Lib/fontTools/feaLib/testdata/spec8a.ttx b/Lib/fontTools/feaLib/testdata/spec8a.ttx
index 21ec2af3a..5b2da5680 100644
--- a/Lib/fontTools/feaLib/testdata/spec8a.ttx
+++ b/Lib/fontTools/feaLib/testdata/spec8a.ttx
@@ -136,7 +136,7 @@
         <!-- LookupType=3 -->
         <LookupFlag value="0"/>
         <!-- SubTableCount=1 -->
-        <AlternateSubst index="0" Format="1">
+        <AlternateSubst index="0">
           <AlternateSet glyph="a">
             <Alternate glyph="a.alt1"/>
             <Alternate glyph="a.alt2"/>