diff --git a/src/nf_modules/bwa/main.nf b/src/nf_modules/bwa/main.nf
index d61f5a95a5455bfeb8fe4908b1171c36cd5edd60..5ae3689383441944863f9c0557f358506c3aed9d 100644
--- a/src/nf_modules/bwa/main.nf
+++ b/src/nf_modules/bwa/main.nf
@@ -52,10 +52,11 @@ process mapping_fastq {
       library = file_id.library
       file_prefix = file_id.id
     } else {
-      library = file_id
-      file_prefix = file_id
+      library = file_id[0]
+      file_prefix = file_id[0]
     }
   } else {
+    library = file_id
     file_prefix = file_id
   }
 bwa_mem_R = "@RG\\tID:${library}\\tSM:${library}\\tLB:lib_${library}\\tPL:illumina"