diff --git a/cwltool/checker.py b/cwltool/checker.py
index 317100af3..d7ccb1c89 100644
--- a/cwltool/checker.py
+++ b/cwltool/checker.py
@@ -406,27 +406,6 @@ def check_all_types(
                         )
                     )
 
-                if is_conditional_step(param_to_step, parm_id):
-                    src_typ = aslist(srcs_of_sink[0]["type"])
-                    snk_typ = sink["type"]
-
-                    if "null" not in src_typ:
-                        src_typ = ["null"] + cast(List[Any], src_typ)
-
-                    if "null" not in cast(
-                        Union[List[str], CWLObjectType], snk_typ
-                    ):  # Given our type names this works even if not a list
-                        validation["warning"].append(
-                            SrcSink(
-                                src_dict[parm_id],
-                                sink,
-                                linkMerge,
-                                message="Source is from conditional step and may produce `null`",
-                            )
-                        )
-
-                    srcs_of_sink[0]["type"] = src_typ
-
             for src in srcs_of_sink:
                 check_result = check_types(src, sink, linkMerge, valueFrom)
                 if check_result == "warning":
diff --git a/cwltool/workflow.py b/cwltool/workflow.py
index 986db5b99..7250b1ef4 100644
--- a/cwltool/workflow.py
+++ b/cwltool/workflow.py
@@ -359,6 +359,8 @@ def __init__(
                     "SubworkflowFeatureRequirement not in requirements"
                 )
 
+        conditional = self.tool.get("when")
+
         if "scatter" in self.tool:
             (feature, _) = self.get_requirement("ScatterFeatureRequirement")
             if not feature:
@@ -398,11 +400,22 @@ def __init__(
             else:
                 nesting = 1
 
-            for _ in range(0, nesting):
+            for n in range(0, nesting):
                 for oparam in outputparms:
+                    if conditional and n == 0:
+                        oparam["type"] = ["null"] + cast(
+                            List[str], aslist(oparam["type"])
+                        )
                     oparam["type"] = {"type": "array", "items": oparam["type"]}
             self.tool["inputs"] = inputparms
             self.tool["outputs"] = outputparms
+        elif conditional:
+            # No scatter, but conditional
+            outputparms = copy.deepcopy(self.tool["outputs"])
+            for oparam in outputparms:
+                oparam["type"] = ["null"] + cast(List[str], aslist(oparam["type"]))
+            self.tool["outputs"] = outputparms
+
         self.prov_obj = None  # type: Optional[ProvenanceProfile]
         if loadingContext.research_obj is not None:
             self.prov_obj = parentworkflowProv