diff --git a/java/com/google/gerrit/server/patch/PatchScriptBuilder.java b/java/com/google/gerrit/server/patch/PatchScriptBuilder.java
index d0b1b66..c2caa01 100644
--- a/java/com/google/gerrit/server/patch/PatchScriptBuilder.java
+++ b/java/com/google/gerrit/server/patch/PatchScriptBuilder.java
@@ -133,9 +133,7 @@
     edits = new ArrayList<>(content.getEdits());
     ImmutableSet<Edit> editsDueToRebase = content.getEditsDueToRebase();
 
-    if (!isModify(content)) {
-      intralineDifferenceIsPossible = false;
-    } else if (diffPrefs.intralineDifference) {
+    if (isModify(content) && diffPrefs.intralineDifference && isIntralineModeAllowed(b)) {
       IntraLineDiff d =
           patchListCache.getIntraLineDiff(
               IntraLineDiffKey.create(a.id, b.id, diffPrefs.ignoreWhitespace),
@@ -274,6 +272,16 @@
     }
   }
 
+  private static boolean isIntralineModeAllowed(Side side) {
+    // The intraline diff cache keys are the same for these cases. It's better to not show
+    // intraline results than showing completely wrong diffs or to run into a server error.
+    return !Patch.isMagic(side.path) && !isSubmoduleCommit(side.mode);
+  }
+
+  private static boolean isSubmoduleCommit(FileMode mode) {
+    return mode.getObjectType() == Constants.OBJ_COMMIT;
+  }
+
   private void correctForDifferencesInNewlineAtEnd() {
     // a.src.size() is the size ignoring a newline at the end whereas a.size() considers it.
     int aSize = a.src.size();
diff --git a/javatests/com/google/gerrit/acceptance/api/revision/RevisionDiffIT.java b/javatests/com/google/gerrit/acceptance/api/revision/RevisionDiffIT.java
index c85e8e4..c27d637 100644
--- a/javatests/com/google/gerrit/acceptance/api/revision/RevisionDiffIT.java
+++ b/javatests/com/google/gerrit/acceptance/api/revision/RevisionDiffIT.java
@@ -19,6 +19,7 @@
 import static com.google.gerrit.extensions.common.testing.DiffInfoSubject.assertThat;
 import static com.google.gerrit.extensions.common.testing.FileInfoSubject.assertThat;
 import static com.google.gerrit.reviewdb.client.Patch.COMMIT_MSG;
+import static com.google.gerrit.reviewdb.client.Patch.MERGE_LIST;
 import static java.util.stream.Collectors.joining;
 import static java.util.stream.Collectors.toMap;
 
@@ -401,6 +402,24 @@
   }
 
   @Test
+  public void diffBetweenPatchSetsOfMergeCommitCanBeRetrievedForCommitMessageAndMergeList()
+      throws Exception {
+    PushOneCommit.Result result = createMergeCommitChange("refs/for/master", "my_file.txt");
+    String changeId = result.getChangeId();
+    String previousPatchSetId = gApi.changes().id(changeId).get().currentRevision;
+    addModifiedPatchSet(changeId, "my_file.txt", content -> content.concat("Line I\nLine II\n"));
+
+    // Call both of them in succession to ensure that they don't share the same cache keys.
+    DiffInfo commitMessageDiffInfo =
+        getDiffRequest(changeId, CURRENT, COMMIT_MSG).withBase(previousPatchSetId).get();
+    DiffInfo mergeListDiffInfo =
+        getDiffRequest(changeId, CURRENT, MERGE_LIST).withBase(previousPatchSetId).get();
+
+    assertThat(commitMessageDiffInfo).content().hasSize(3);
+    assertThat(mergeListDiffInfo).content().hasSize(1);
+  }
+
+  @Test
   public void diffOfUnmodifiedFileMarksAllLinesAsCommon() throws Exception {
     String filePath = "a_new_file.txt";
     String fileContent = "Line 1\nLine 2\nLine 3\n";