diff --git a/pom.xml b/pom.xml
index 485abbf..21763c5 100644
--- a/pom.xml
+++ b/pom.xml
@@ -58,6 +58,12 @@
             <version>1703.vd5a_2b_29c6cdc</version>
         </dependency>
 
+        <dependency>
+            <groupId>com.github.tomakehurst</groupId>
+            <artifactId>wiremock-jre8-standalone</artifactId>
+            <version>2.35.2</version>
+            <scope>test</scope>
+        </dependency>
 
         <!-- to resolve conflict in github-branch-source -->
         <dependency>
diff --git a/src/main/java/org/jenkinsci/plugins/pipeline/github/PullRequestGroovyObject.java b/src/main/java/org/jenkinsci/plugins/pipeline/github/PullRequestGroovyObject.java
index 9416635..83cdecc 100644
--- a/src/main/java/org/jenkinsci/plugins/pipeline/github/PullRequestGroovyObject.java
+++ b/src/main/java/org/jenkinsci/plugins/pipeline/github/PullRequestGroovyObject.java
@@ -283,7 +283,7 @@ public int getChangedFiles() {
     }
 
     @Whitelisted
-    public boolean isMergeable() {
+    public Boolean isMergeable() { // Boolean instead of boolean because this can be null!
         return pullRequest.isMergeable();
     }
 
diff --git a/src/test/java/org/jenkinsci/plugins/pipeline/github/PullRequestGroovyObjectTest.java b/src/test/java/org/jenkinsci/plugins/pipeline/github/PullRequestGroovyObjectTest.java
index f678832..a0fc645 100644
--- a/src/test/java/org/jenkinsci/plugins/pipeline/github/PullRequestGroovyObjectTest.java
+++ b/src/test/java/org/jenkinsci/plugins/pipeline/github/PullRequestGroovyObjectTest.java
@@ -1,10 +1,21 @@
 package org.jenkinsci.plugins.pipeline.github;
 
+import static org.junit.Assert.assertEquals;
+
+import org.jenkinsci.plugins.github_branch_source.GitHubSCMSource;
+import org.jenkinsci.plugins.github_branch_source.PullRequestSCMHead;
 import org.jenkinsci.plugins.workflow.job.WorkflowJob;
 import org.junit.Rule;
 import org.junit.Test;
 import org.jvnet.hudson.test.JenkinsRule;
 
+import static com.github.tomakehurst.wiremock.client.WireMock.*;
+import com.github.tomakehurst.wiremock.junit.WireMockRule;
+
+import hudson.model.Item;
+import jenkins.scm.api.SCMHead;
+import jenkins.scm.api.SCMSource;
+
 /**
  * @author Aaron Whiteside
  */
@@ -13,10 +24,61 @@ public class PullRequestGroovyObjectTest {
     @Rule
     public JenkinsRule r = new JenkinsRule();
 
+    @Rule
+    public WireMockRule wireMockRule = new WireMockRule();
+
+    public static class TestHeadByItemImpl extends SCMHead.HeadByItem {
+        @Override
+        public SCMHead getHead(Item item) {
+            return new PullRequestSCMHead("PR-42", "owner", "repo", null, 42, null, null, null);
+        }
+    }
+
+    public static class TestSourceByItemImpl extends SCMSource.SourceByItem {
+        public TestSourceByItemImpl(int port) {
+            this.port = port;
+        }
+
+        @Override
+        public SCMSource getSource(Item item) {
+            GitHubSCMSource result = new GitHubSCMSource("owner", "repo", null, false);
+            result.setCredentialsId("credentialsId");
+            result.setApiUri(String.format("http://localhost:%d", port));
+            return result;
+        }
+
+        private int port;
+    }
+
     @Test
     public void testReadOnlyProperties() throws Exception {
         WorkflowJob job = r.createProject(WorkflowJob.class, "p");
         r.getInstance().getAllItems().forEach(System.out::println);
     }
 
+    @Test
+    public void testPullRequestMergeabilityUndecided() throws Exception {
+        WorkflowJob job = r.createProject(WorkflowJob.class, "p");
+        r.jenkins.getExtensionList(SCMHead.HeadByItem.class).add(new TestHeadByItemImpl());
+        r.jenkins.getExtensionList(SCMSource.SourceByItem.class).add(new TestSourceByItemImpl(wireMockRule.port()));
+        stubFor(get(urlPathMatching("/api/v3/repos/owner/repo/pulls/42"))
+            .willReturn(aResponse()
+                .withStatus(200)
+                .withBody("{}")));
+        PullRequestGroovyObject prgo = new PullRequestGroovyObject((WorkflowJob)job);
+        assertEquals(null, prgo.isMergeable());
+   }
+
+   @Test
+   public void testPullRequestMergeable() throws Exception {
+       WorkflowJob job = r.createProject(WorkflowJob.class, "p");
+       r.jenkins.getExtensionList(SCMHead.HeadByItem.class).add(new TestHeadByItemImpl());
+       r.jenkins.getExtensionList(SCMSource.SourceByItem.class).add(new TestSourceByItemImpl(wireMockRule.port()));
+       stubFor(get(urlPathMatching("/api/v3/repos/owner/repo/pulls/42"))
+           .willReturn(aResponse()
+               .withStatus(200)
+               .withBody("{\"mergeable\":\"true\"}")));
+       PullRequestGroovyObject prgo = new PullRequestGroovyObject((WorkflowJob)job);
+       assertEquals(true, prgo.isMergeable());
+  }
 }
\ No newline at end of file