diff --git a/.ci/build.sh b/.ci/build.sh
index e73e540b52..0c58fd5138 100755
--- a/.ci/build.sh
+++ b/.ci/build.sh
@@ -21,12 +21,22 @@ if [[ "$GHC_HEAD" != "yes" ]]; then
   mv cabal.project.local.disabled cabal.project.local
   [[ ! -f cabal.project.freeze.disabled ]] || mv cabal.project.freeze.disabled cabal.project.freeze
 fi
-set -u
 
-cabal v2-build clash-prelude --write-ghc-environment-files=always
-# Undo mmap crash workaround for clash-cosim, see PR #2572.
-# It uses a custom setup, which can't be linked with -rtsopts
-GHCRTS="" cabal v2-build clash-cosim --write-ghc-environment-files=always
+# Any package with a custom setup or using happy can only be build with GHCRTS=""
+# see PR #2572.
+if [[ "$GHCRTS" != "" ]]; then
+CANT_BUILD_WITH_RTSOPTS="ghc-paths haskell-src-exts pretty-show singletons"
+
+GHC_MAJOR_VERSION=$(ghc --numeric-version | awk -F . '{print $1}')
+if [[ "$GHC_MAJOR_VERSION" -gt "8" ]]; then
+CANT_BUILD_WITH_RTSOPTS="$CANT_BUILD_WITH_RTSOPTS singletons-base"
+fi
+cabal v2-build --only-dependencies $CANT_BUILD_WITH_RTSOPTS
+GHCRTS="" cabal v2-build $CANT_BUILD_WITH_RTSOPTS
+cabal v2-build clash-prelude
+GHCRTS="" cabal v2-build clash-cosim
+fi
+set -u
 
 # Build with default constraints
 cabal v2-build all --write-ghc-environment-files=always