diff --git a/Library/Homebrew/sandbox.rb b/Library/Homebrew/sandbox.rb
index ab94abc9f8..a389505b0f 100644
--- a/Library/Homebrew/sandbox.rb
+++ b/Library/Homebrew/sandbox.rb
@@ -101,8 +101,21 @@ class Sandbox
       command = [SANDBOX_EXEC, "-f", seatbelt.path, *args]
       # Start sandbox in a pseudoterminal to prevent access of the parent terminal.
       T.unsafe(PTY).spawn(*command) do |r, w, pid|
-        old_winch = trap(:WINCH) { w.winsize = $stdout.winsize if $stdout.tty? }
-        w.winsize = $stdout.winsize if $stdout.tty?
+        # Set the PTY's window size to match the parent terminal.
+        # Some formula tests are sensitive to the terminal size and fail if this is not set.
+        winch = proc do |_sig|
+          w.winsize = if $stdout.tty?
+            # We can only use IO#winsize if the IO object is a TTY.
+            $stdout.winsize
+          else
+            # Otherwise, default to tput, if available.
+            # This relies on ncurses rather than the system's ioctl.
+            [Utils.popen_read("tput", "lines").to_i, Utils.popen_read("tput", "cols").to_i]
+          end
+        end
+        # Update the window size whenever the parent terminal's window size changes.
+        old_winch = trap(:WINCH, &winch)
+        winch.call(nil)
 
         $stdin.raw! if $stdin.tty?
         stdin_thread = Thread.new { IO.copy_stream($stdin, w) }
diff --git a/Library/Homebrew/test/sandbox_spec.rb b/Library/Homebrew/test/sandbox_spec.rb
index 2d827f9e06..46b3a57df5 100644
--- a/Library/Homebrew/test/sandbox_spec.rb
+++ b/Library/Homebrew/test/sandbox_spec.rb
@@ -34,7 +34,8 @@ describe Sandbox, :needs_macos do
     it "complains on failure" do
       ENV["HOMEBREW_VERBOSE"] = "1"
 
-      expect(Utils).to receive(:popen_read).and_return("foo")
+      allow(Utils).to receive(:popen_read).and_call_original
+      allow(Utils).to receive(:popen_read).with("syslog", any_args).and_return("foo")
 
       expect { sandbox.exec "false" }
         .to raise_error(ErrorDuringExecution)
@@ -49,7 +50,8 @@ describe Sandbox, :needs_macos do
         Mar 17 02:55:06 sandboxd[342]: Python(49765) deny file-write-unlink /System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/distutils/errors.pyc
         bar
       EOS
-      expect(Utils).to receive(:popen_read).and_return(with_bogus_error)
+      allow(Utils).to receive(:popen_read).and_call_original
+      allow(Utils).to receive(:popen_read).with("syslog", any_args).and_return(with_bogus_error)
 
       expect { sandbox.exec "false" }
         .to raise_error(ErrorDuringExecution)