diff --git a/pack.h b/pack.h
index 4814800..790ff4a 100644
--- a/pack.h
+++ b/pack.h
@@ -15,5 +15,33 @@
 	unsigned int hdr_entries;
 };
 
+/*
+ * Packed object index header
+ *
+ * struct pack_idx_header {
+ * 	uint32_t idx_signature;
+ *	uint32_t idx_version;
+ * };
+ *
+ * Note: this header isn't active yet.  In future versions of git
+ * we may change the index file format.  At that time we would start
+ * the first four bytes of the new index format with this signature,
+ * as all older git binaries would find this value illegal and abort
+ * reading the file.
+ *
+ * This is the case because the number of objects in a packfile
+ * cannot exceed 1,431,660,000 as every object would need at least
+ * 3 bytes of data and the overall packfile cannot exceed 4 GiB due
+ * to the 32 bit offsets used by the index.  Clearly the signature
+ * exceeds this maximum.
+ *
+ * Very old git binaries will also compare the first 4 bytes to the
+ * next 4 bytes in the index and abort with a "non-monotonic index"
+ * error if the second 4 byte word is smaller than the first 4
+ * byte word.  This would be true in the proposed future index
+ * format as idx_signature would be greater than idx_version.
+ */
+#define PACK_IDX_SIGNATURE 0xff744f63	/* "\377tOc" */
+
 extern int verify_pack(struct packed_git *, int);
 #endif
diff --git a/sha1_file.c b/sha1_file.c
index 1b1c0f7..0b70545 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -456,12 +456,23 @@
 
 	/* check index map */
 	if (idx_size < 4*256 + 20 + 20)
-		return error("index file too small");
+		return error("index file %s is too small", path);
+
+	/* a future index format would start with this, as older git
+	 * binaries would fail the non-monotonic index check below.
+	 * give a nicer warning to the user if we can.
+	 */
+	if (index[0] == htonl(PACK_IDX_SIGNATURE))
+		return error("index file %s is a newer version"
+			" and is not supported by this binary"
+			" (try upgrading GIT to a newer version)",
+			path);
+
 	nr = 0;
 	for (i = 0; i < 256; i++) {
 		unsigned int n = ntohl(index[i]);
 		if (n < nr)
-			return error("non-monotonic index");
+			return error("non-monotonic index %s", path);
 		nr = n;
 	}
 
@@ -473,7 +484,7 @@
 	 *  - 20-byte SHA1 file checksum
 	 */
 	if (idx_size != 4*256 + nr * 24 + 20 + 20)
-		return error("wrong index file size");
+		return error("wrong index file size in %s", path);
 
 	return 0;
 }