diff --git a/build-constraints.yaml b/build-constraints.yaml
index 5b0bb072..83b92edd 100644
--- a/build-constraints.yaml
+++ b/build-constraints.yaml
@@ -6028,6 +6028,8 @@ expected-benchmark-failures:
     - thyme
     - xmlgen # https://github.com/skogsbaer/xmlgen/issues/6
 
+    # https://github.com/mmark-md/mmark-ext/issues/20
+    - mmark-ext
 # end of expected-benchmark-failures