diff --git a/libfprint/fpi-assembling.c b/libfprint/fpi-assembling.c
index 3eabb4d0..e33dafe5 100644
--- a/libfprint/fpi-assembling.c
+++ b/libfprint/fpi-assembling.c
@@ -419,7 +419,7 @@ struct fp_img *fpi_assemble_lines(struct fpi_line_asmbl_ctx *ctx,
 	struct fp_img *img;
 
 	g_return_val_if_fail (lines != NULL, NULL);
-	g_return_val_if_fail (num_lines > 0, NULL);
+	g_return_val_if_fail (num_lines >= 2, NULL);
 
 	fp_dbg("%"G_GINT64_FORMAT, g_get_real_time());