call toString on value

src/main/java/org/jenkinsci/plugins/workflow/steps/EnvStep.java

@@ -178,7 +178,7 @@ public Map<String, Object> customInstantiate(@NonNull Map<String, Object> argume
         private static List<String> toKeyValueList(Map<?, ?> map) {
             return map.entrySet().stream()
                 .filter(e -> e.getKey() instanceof String)
-                .collect(Collectors.toMap(e -> (String)e.getKey(), e -> e.getValue() == null ? "" : (String) e.getValue()))
+                .collect(Collectors.toMap(e -> (String)e.getKey(), e -> e.getValue() == null ? "" : e.getValue().toString()))
                 .entrySet().stream()
                 .map(m -> m.getKey() + "=" + m.getValue())
                 .collect(Collectors.toList());

src/test/java/org/jenkinsci/plugins/workflow/steps/EnvStepTest.java

@@ -245,6 +245,26 @@ public class EnvStepTest {
         });
     }
 
+    @Test public void mapNumbersNested() {
+        story.addStep(new Statement() {
+            @Override public void evaluate() throws Throwable {
+                WorkflowJob p = story.j.jenkins.createProject(WorkflowJob.class, "p");
+                p.setDefinition(new CpsFlowDefinition(
+                    "node {\n" +
+                        "  withEnv([A: 1]) {\n" +
+                        "    withEnv([B: 2]) {\n" +
+                        "      withEnv([C: true]) {\n" +
+                        "        isUnix() ? sh('echo A=$A B=$B C=$C') : bat('echo A=%A% B=%B% C=%C%')\n" +
+                        "      }\n" +
+                        "    }\n" +
+                        "  }\n" +
+                        "}", true));
+                WorkflowRun b = story.j.assertBuildStatusSuccess(p.scheduleBuild2(0));
+                story.j.assertLogContains("A=1 B=2 C=true", b);
+            }
+        });
+    }
+
     @Test public void mapNested() {
         story.addStep(new Statement() {
             @Override public void evaluate() throws Throwable {