diff --git a/modules/ngscheckmate/ncm/main.nf b/modules/ngscheckmate/ncm/main.nf
index 2712c984..0898cee8 100644
--- a/modules/ngscheckmate/ncm/main.nf
+++ b/modules/ngscheckmate/ncm/main.nf
@@ -12,7 +12,7 @@ process NGSCHECKMATE_NCM {
     path fasta
 
     output:
-    path "*.pdf"            , emit: pdf
+    path "*.pdf"            , emit: pdf, optional: true
     path "*_corr_matrix.txt", emit: corr_matrix
     path "*_matched.txt"    , emit: matched
     path "*_all.txt"        , emit: all