diff --git a/Lib/fontTools/feaLib/builder.py b/Lib/fontTools/feaLib/builder.py
index a378b3794..6758c08d5 100644
--- a/Lib/fontTools/feaLib/builder.py
+++ b/Lib/fontTools/feaLib/builder.py
@@ -1356,8 +1356,9 @@ class Builder(object):
                 "Empty glyph class in contextual substitution", location
             )
         # https://github.com/fonttools/fonttools/issues/512
+        # https://github.com/fonttools/fonttools/issues/2150
         chain = self.get_lookup_(location, ChainContextSubstBuilder)
-        sub = chain.find_chainable_single_subst(set(mapping.keys()))
+        sub = chain.find_chainable_single_subst(mapping)
         if sub is None:
             sub = self.get_chained_lookup_(location, SingleSubstBuilder)
         sub.mapping.update(mapping)
diff --git a/Lib/fontTools/otlLib/builder.py b/Lib/fontTools/otlLib/builder.py
index b4ab36842..a72aa9fdb 100644
--- a/Lib/fontTools/otlLib/builder.py
+++ b/Lib/fontTools/otlLib/builder.py
@@ -764,7 +764,7 @@ class ChainContextSubstBuilder(ChainContextualBuilder):
                             result.setdefault(glyph, set()).update(replacements)
         return result
 
-    def find_chainable_single_subst(self, glyphs):
+    def find_chainable_single_subst(self, mapping):
         """Helper for add_single_subst_chained_()"""
         res = None
         for rule in self.rules[::-1]:
@@ -772,7 +772,7 @@ class ChainContextSubstBuilder(ChainContextualBuilder):
                 return res
             for sub in rule.lookups:
                 if isinstance(sub, SingleSubstBuilder) and not any(
-                    g in glyphs for g in sub.mapping.keys()
+                    g in mapping and mapping[g] != sub.mapping[g] for g in sub.mapping
                 ):
                     res = sub
         return res
diff --git a/Tests/feaLib/data/bug512.ttx b/Tests/feaLib/data/bug512.ttx
index 693ebeb7c..4ad3af2f9 100644
--- a/Tests/feaLib/data/bug512.ttx
+++ b/Tests/feaLib/data/bug512.ttx
@@ -73,7 +73,7 @@
               <!-- SubstCount=1 -->
               <SubstLookupRecord index="0">
                 <SequenceIndex value="0"/>
-                <LookupListIndex value="2"/>
+                <LookupListIndex value="1"/>
               </SubstLookupRecord>
             </SubRule>
           </SubRuleSet>
@@ -94,7 +94,6 @@
         <!-- SubTableCount=1 -->
         <SingleSubst index="0">
           <Substitution in="G" out="g"/>
-          <Substitution in="H" out="H.swash"/>
         </SingleSubst>
       </Lookup>
     </LookupList>