diff --git a/src/liblzma/common/index.c b/src/liblzma/common/index.c
index 24ec3c10..449d937f 100644
--- a/src/liblzma/common/index.c
+++ b/src/liblzma/common/index.c
@@ -660,6 +660,12 @@ lzma_index_append(lzma_index *i, const lzma_allocator *allocator,
 	if (uncompressed_base + uncompressed_size > LZMA_VLI_MAX)
 		return LZMA_DATA_ERROR;
 
+	// Check that the new unpadded sum will not overflow. This is
+	// checked again in index_file_size(), but the unpadded sum is
+	// passed to vli_ceil4() which expects a valid lzma_vli value.
+	if (compressed_base + unpadded_size > UNPADDED_SIZE_MAX)
+		return LZMA_DATA_ERROR;
+
 	// Check that the file size will stay within limits.
 	if (index_file_size(s->node.compressed_base,
 			compressed_base + unpadded_size, s->record_count + 1,