diff --git a/small_RNA-seq/small_RNA-seq.snakefile b/small_RNA-seq/small_RNA-seq.snakefile
index 9666f4d5cb4df60b53772f421e3359bd80663ca2..ab9838e7282b2c2e2f8f195483b19fa3c70a5984 100644
--- a/small_RNA-seq/small_RNA-seq.snakefile
+++ b/small_RNA-seq/small_RNA-seq.snakefile
@@ -2059,7 +2059,9 @@ def source_small_RNA_counts(wildcards):
 def make_tag_association(dfs, tag):
     """Associates a tag "tag" to the union of the indices of dataframes *dfs*."""
     idx = reduce(union, (df.index for df in dfs))
-    return pd.DataFrame(list(zip(idx, repeat(tag)))).set_index(0)
+    if len(idx):
+        return pd.DataFrame(list(zip(idx, repeat(tag)))).set_index(0)
+    return pd.DataFrame()
 
 
 # Could we use biotype instead?