diff --git a/scm-plugins/scm-git-plugin/src/main/java/sonia/scm/repository/GitUtil.java b/scm-plugins/scm-git-plugin/src/main/java/sonia/scm/repository/GitUtil.java
index 7175d3b646..992e595382 100644
--- a/scm-plugins/scm-git-plugin/src/main/java/sonia/scm/repository/GitUtil.java
+++ b/scm-plugins/scm-git-plugin/src/main/java/sonia/scm/repository/GitUtil.java
@@ -70,6 +70,7 @@ import java.util.Optional;
 import java.util.concurrent.TimeUnit;
 
 import static java.util.Optional.of;
+import static java.util.Optional.ofNullable;
 
 //~--- JDK imports ------------------------------------------------------------
 
@@ -722,12 +723,13 @@ public final class GitUtil
   /**
    * Computes the first common ancestor of two revisions, aka merge base.
    */
-  public static ObjectId computeCommonAncestor(org.eclipse.jgit.lib.Repository repository, ObjectId revision1, ObjectId revision2) throws IOException {
+  public static Optional<ObjectId> computeCommonAncestor(org.eclipse.jgit.lib.Repository repository, ObjectId revision1, ObjectId revision2) throws IOException {
     try (RevWalk mergeBaseWalk = new RevWalk(repository)) {
       mergeBaseWalk.setRevFilter(RevFilter.MERGE_BASE);
       mergeBaseWalk.markStart(mergeBaseWalk.lookupCommit(revision1));
       mergeBaseWalk.markStart(mergeBaseWalk.parseCommit(revision2));
-      return mergeBaseWalk.next().getId();
+      RevCommit commonAncestor = mergeBaseWalk.next();
+      return ofNullable(commonAncestor).map(RevCommit::getId);
     }
   }
 
diff --git a/scm-plugins/scm-git-plugin/src/main/java/sonia/scm/repository/spi/Differ.java b/scm-plugins/scm-git-plugin/src/main/java/sonia/scm/repository/spi/Differ.java
index ca417550f4..81d19886c3 100644
--- a/scm-plugins/scm-git-plugin/src/main/java/sonia/scm/repository/spi/Differ.java
+++ b/scm-plugins/scm-git-plugin/src/main/java/sonia/scm/repository/spi/Differ.java
@@ -10,11 +10,15 @@ import org.eclipse.jgit.revwalk.RevWalk;
 import org.eclipse.jgit.treewalk.EmptyTreeIterator;
 import org.eclipse.jgit.treewalk.TreeWalk;
 import org.eclipse.jgit.treewalk.filter.PathFilter;
+import sonia.scm.BadRequestException;
 import sonia.scm.repository.GitUtil;
 import sonia.scm.util.Util;
 
 import java.io.IOException;
 import java.util.List;
+import java.util.Optional;
+
+import static java.util.Collections.emptyList;
 
 final class Differ implements AutoCloseable {
 
@@ -55,7 +59,9 @@ final class Differ implements AutoCloseable {
       if (!Strings.isNullOrEmpty(request.getAncestorChangeset()))
       {
         ObjectId otherRevision = repository.resolve(request.getAncestorChangeset());
-        ObjectId ancestorId = computeCommonAncestor(repository, revision, otherRevision);
+        ObjectId ancestorId =
+          computeCommonAncestor(repository, revision, otherRevision)
+            .orElseThrow(NoCommonHistoryException::new);
         RevTree tree = walk.parseCommit(ancestorId).getTree();
         treeWalk.addTree(tree);
       }
@@ -82,7 +88,7 @@ final class Differ implements AutoCloseable {
     return new Differ(commit, walk, treeWalk);
   }
 
-  private static ObjectId computeCommonAncestor(org.eclipse.jgit.lib.Repository repository, ObjectId revision1, ObjectId revision2) throws IOException {
+  private static Optional<ObjectId> computeCommonAncestor(org.eclipse.jgit.lib.Repository repository, ObjectId revision1, ObjectId revision2) throws IOException {
     return GitUtil.computeCommonAncestor(repository, revision1, revision2);
   }
 
@@ -115,4 +121,16 @@ final class Differ implements AutoCloseable {
       return entries;
     }
   }
+
+  private static class NoCommonHistoryException extends BadRequestException {
+
+    private NoCommonHistoryException() {
+      super(emptyList(), "no common history");
+    }
+
+    @Override
+    public String getCode() {
+      return "4iRct4avG1";
+    }
+  }
 }
diff --git a/scm-webapp/src/main/resources/locales/de/plugins.json b/scm-webapp/src/main/resources/locales/de/plugins.json
index 9c5bb3f5f5..ef6d5d8c96 100644
--- a/scm-webapp/src/main/resources/locales/de/plugins.json
+++ b/scm-webapp/src/main/resources/locales/de/plugins.json
@@ -175,6 +175,10 @@
     "40RaYIeeR1": {
       "displayName": "Es wurden keine Änderungen durchgeführt",
       "description": "Das Repository wurde nicht verändert. Daher konnte kein neuer Commit erzeugt werden."
+    },
+    "4iRct4avG1": {
+      "displayName": "Die Revisionen haben keinen gemeinsamen Ursprung",
+      "description": "Die Historie der Revisionen hat keinen gemeinsamen Urspung und kann somit auch nicht gegen einen solchen verglichen werden."
     }
   },
   "namespaceStrategies": {
diff --git a/scm-webapp/src/main/resources/locales/en/plugins.json b/scm-webapp/src/main/resources/locales/en/plugins.json
index 1ffffd73b7..247fa212f3 100644
--- a/scm-webapp/src/main/resources/locales/en/plugins.json
+++ b/scm-webapp/src/main/resources/locales/en/plugins.json
@@ -175,6 +175,10 @@
     "40RaYIeeR1": {
       "displayName": "No changes were made",
       "description": "No changes were made to the files of the repository. Therefor no new commit could be created."
+    },
+    "4iRct4avG1": {
+      "displayName": "The revisions have unrelated histories",
+      "description": "The revisions have unrelated histories. Therefor there is no common commit to compare with."
     }
   },
   "namespaceStrategies": {