diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/commitgraph/CommitGraphWriter.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/commitgraph/CommitGraphWriter.java
index c5cfe9586..bab262231 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/commitgraph/CommitGraphWriter.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/commitgraph/CommitGraphWriter.java
@@ -222,6 +222,9 @@ private void writeChunks(ProgressMonitor monitor,
 				}
 				chunk.data.get().writeTo(out);
 				break;
+			default:
+				throw new IllegalStateException(
+						"Don't know how to write chunk " + chunkId); //$NON-NLS-1$
 			}
 		}
 	}