diff --git a/Lib/fontTools/feaLib/parser.py b/Lib/fontTools/feaLib/parser.py
index bda34e261..1008f16a3 100644
--- a/Lib/fontTools/feaLib/parser.py
+++ b/Lib/fontTools/feaLib/parser.py
@@ -826,8 +826,14 @@ class Parser(object):
                 'is not supported',
                 location)
 
+        # If there are remaining glyphs to parse, this is an invalid GSUB statement
+        if len(new) != 0:
+            raise FeatureLibError(
+                'Invalid substitution statement',
+                location
+            )
+
         # GSUB lookup type 6: Chaining contextual substitution.
-        assert len(new) == 0, new
         rule = self.ast.ChainContextSubstStatement(
             old_prefix, old, old_suffix, lookups, location=location)
         return rule
diff --git a/Tests/feaLib/data/GSUB_error.fea b/Tests/feaLib/data/GSUB_error.fea
new file mode 100644
index 000000000..ae175c501
--- /dev/null
+++ b/Tests/feaLib/data/GSUB_error.fea
@@ -0,0 +1,13 @@
+# Trigger a parser error in the function parse_substitute_ in order to improve the error message.
+# Note that this is not a valid substitution, this test is made to trigger an error.
+
+languagesystem latn dflt;
+
+@base = [a e];
+@accents = [acute grave];
+
+feature abvs {
+    lookup lookup1 {
+        sub @base @accents by @base;
+    } lookup1;
+} abvs;
diff --git a/Tests/feaLib/parser_test.py b/Tests/feaLib/parser_test.py
index cb4e689b7..f2256b4c4 100644
--- a/Tests/feaLib/parser_test.py
+++ b/Tests/feaLib/parser_test.py
@@ -1502,6 +1502,13 @@ class ParserTest(unittest.TestCase):
             FeatureLibError,
             'Expected "by", "from" or explicit lookup references',
             self.parse, "feature liga {substitute f f i;} liga;")
+    
+    def test_substitute_invalid_statement(self):
+        self.assertRaisesRegex(
+            FeatureLibError,
+            "Invalid substitution statement",
+            Parser(self.getpath("GSUB_error.fea"), GLYPHNAMES).parse
+        )
 
     def test_subtable(self):
         doc = self.parse("feature test {subtable;} test;")