diff --git a/conformance_tests.yaml b/conformance_tests.yaml
index 8d397d7..f68a976 100644
--- a/conformance_tests.yaml
+++ b/conformance_tests.yaml
@@ -3525,3 +3525,9 @@
       an_array_of_mixed_booleans: [ false, true, false ]
       an_array_of_trues: [ true, true, true ]
       an_int: 42
+
+- id: invalid_expression_tool_output
+  tool: tests/null-expression4-tool.cwl
+  doc: Test that the output objects returned by an ExpressionTool process are validated against the outputs schema.
+  should_fail: true
+  tags: [ inline_javascript, expression_tool ]
\ No newline at end of file
diff --git a/tests/null-expression3-tool.cwl b/tests/null-expression3-tool.cwl
index 9d8da1a..14dc88b 100644
--- a/tests/null-expression3-tool.cwl
+++ b/tests/null-expression3-tool.cwl
@@ -7,6 +7,6 @@ cwlVersion: v1.3.0-dev1
 inputs: []
 
 outputs:
-  output: Any
+  output: Any?
 
 expression: "$({'output': null })"
diff --git a/tests/null-expression4-tool.cwl b/tests/null-expression4-tool.cwl
new file mode 100644
index 0000000..9d8da1a
--- /dev/null
+++ b/tests/null-expression4-tool.cwl
@@ -0,0 +1,12 @@
+#!/usr/bin/env cwl-runner
+
+class: ExpressionTool
+requirements: { InlineJavascriptRequirement: {} }
+cwlVersion: v1.3.0-dev1
+
+inputs: []
+
+outputs:
+  output: Any
+
+expression: "$({'output': null })"