diff --git a/extract.c b/extract.c
index d6214f70..a6492716 100644
--- a/extract.c
+++ b/extract.c
@@ -69,7 +69,7 @@ extract_finish(struct extraction_context *ctx, char **text, size_t *len)
         /* Selection of empty cells only */
         if (!ensure_size(ctx, 1))
             goto out;
-        ctx->buf[ctx->idx] = L'\0';
+        ctx->buf[ctx->idx++] = L'\0';
     } else {
         assert(ctx->idx > 0);
         assert(ctx->idx <= ctx->size);
@@ -78,7 +78,7 @@ extract_finish(struct extraction_context *ctx, char **text, size_t *len)
         else {
             if (!ensure_size(ctx, 1))
                 goto out;
-            ctx->buf[ctx->idx] = L'\0';
+            ctx->buf[ctx->idx++] = L'\0';
         }
     }