diff --git a/Library/Homebrew/test/diagnostic_spec.rb b/Library/Homebrew/test/diagnostic_spec.rb
index 058b9823ca..8a25413ed4 100644
--- a/Library/Homebrew/test/diagnostic_spec.rb
+++ b/Library/Homebrew/test/diagnostic_spec.rb
@@ -12,6 +12,12 @@ describe Homebrew::Diagnostic::Checks do
       .to match("Some directories in your path end in a slash")
   end
 
+  specify "#check_build_from_source" do
+    ENV["HOMEBREW_BUILD_FROM_SOURCE"] = "1"
+    expect(subject.check_build_from_source)
+      .to match("You have HOMEBREW_BUILD_FROM_SOURCE set.")
+  end
+
   specify "#check_for_anaconda" do
     mktmpdir do |path|
       anaconda = "#{path}/anaconda"