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 bab262231..afbd7e654 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
@@ -122,6 +122,7 @@ public Stats write(@NonNull ProgressMonitor monitor,
 		}
 
 		List<ChunkHeader> chunks = createChunks(stats);
+		long expectedSize = calculateExpectedSize(chunks);
 		long writeCount = 256 + 2 * graphCommits.size()
 				+ graphCommits.getExtraEdgeCnt();
 		monitor.beginTask(
@@ -135,6 +136,11 @@ public Stats write(@NonNull ProgressMonitor monitor,
 			writeChunkLookup(out, chunks);
 			writeChunks(monitor, out, chunks);
 			writeCheckSum(out);
+			if (expectedSize != out.length()) {
+				throw new IllegalStateException(String.format(
+						"Commit-graph: expected %d bytes but out has %d bytes", //$NON-NLS-1$
+						expectedSize, out.length()));
+			}
 		} catch (InterruptedIOException e) {
 			throw new IOException(JGitText.get().commitGraphWritingCancelled,
 					e);
@@ -168,6 +174,12 @@ private List<ChunkHeader> createChunks(Stats stats)
 		return chunks;
 	}
 
+	private static long calculateExpectedSize(List<ChunkHeader> chunks) {
+		int chunkLookup = (chunks.size() + 1) * CHUNK_LOOKUP_WIDTH;
+		long chunkContent = chunks.stream().mapToLong(c -> c.size).sum();
+		return /* header */ 8 + chunkLookup + chunkContent + /* CRC */ 20;
+	}
+
 	private void writeHeader(CancellableDigestOutputStream out, int numChunks)
 			throws IOException {
 		byte[] headerBuffer = new byte[8];