diff --git a/modules/diamond/blastp/main.nf b/modules/diamond/blastp/main.nf
index fc77ee7b..033186ea 100644
--- a/modules/diamond/blastp/main.nf
+++ b/modules/diamond/blastp/main.nf
@@ -38,6 +38,11 @@ process DIAMOND_BLASTP {
         case "sam": outfmt = 101; break
         case "tsv": outfmt = 102; break
         case "paf": outfmt = 103; break
+        default:
+            outfmt = '6';
+            out_ext = 'txt';
+            log.warn("Unknown output file format provided (${out_ext}): selecting DIAMOND default of tabular BLAST output (txt)");
+            break
     }
     """
     DB=`find -L ./ -name "*.dmnd" | sed 's/.dmnd//'`
diff --git a/modules/diamond/blastx/main.nf b/modules/diamond/blastx/main.nf
index 3479eedb..d3272279 100644
--- a/modules/diamond/blastx/main.nf
+++ b/modules/diamond/blastx/main.nf
@@ -38,6 +38,11 @@ process DIAMOND_BLASTX {
         case "sam": outfmt = 101; break
         case "tsv": outfmt = 102; break
         case "paf": outfmt = 103; break
+        default:
+            outfmt = '6';
+            out_ext = 'txt';
+            log.warn("Unknown output file format provided (${out_ext}): selecting DIAMOND default of tabular BLAST output (txt)");
+            break
     }
     """
     DB=`find -L ./ -name "*.dmnd" | sed 's/.dmnd//'`
diff --git a/tests/modules/diamond/blastx/main.nf b/tests/modules/diamond/blastx/main.nf
index bb1b55a8..847a64b1 100644
--- a/tests/modules/diamond/blastx/main.nf
+++ b/tests/modules/diamond/blastx/main.nf
@@ -9,7 +9,7 @@ workflow test_diamond_blastx {
 
     db = [ file(params.test_data['sarscov2']['genome']['proteome_fasta'], checkIfExists: true) ]
     fasta = [ file(params.test_data['sarscov2']['genome']['transcriptome_fasta'], checkIfExists: true) ]
-    out_ext = 'txt'
+    out_ext = 'tfdfdt'
     blast_columns = 'qseqid qlen'
 
     DIAMOND_MAKEDB ( db )