diff --git a/cwltool/singularity.py b/cwltool/singularity.py
index 40d1bc9b5..5e5b72157 100644
--- a/cwltool/singularity.py
+++ b/cwltool/singularity.py
@@ -134,10 +134,14 @@ def is_version_3_10_or_newer() -> bool:
 
 
 def _normalize_image_id(string: str) -> str:
+    if ":" not in string:
+        string+="_latest"
     return string.replace("/", "_") + ".img"
 
 
 def _normalize_sif_id(string: str) -> str:
+    if ":" not in string:
+        string+="_latest"
     return string.replace("/", "_") + ".sif"