diff --git a/govuk_tech_docs.gemspec b/govuk_tech_docs.gemspec
index 8404553f..d4ce2eea 100644
--- a/govuk_tech_docs.gemspec
+++ b/govuk_tech_docs.gemspec
@@ -45,7 +45,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency "chronic"
   spec.add_dependency "concurrent-ruby"
   spec.add_dependency "csv" # TODO: remove once tilt declares this itself.
-  spec.add_dependency "haml", "~> 6.0" # middleman-core (4.6.3) depends on haml (>= 4.0.5, < 7)
+  spec.add_dependency "haml", ">= 6", "< 8" # middleman-core (4.6.3) depends on haml (>= 4.0.5, < 7)
   spec.add_dependency "middleman"
   spec.add_dependency "middleman-autoprefixer"
   spec.add_dependency "middleman-compass"