diff --git a/tools/exekall/src/exekall/_utils.py b/tools/exekall/src/exekall/_utils.py
index 5c49dd94b6719f26538f9c05eb459bbba4618597..05a5d240ca896404b45ec6a7daf68cce9b06f4d4 100644
--- a/tools/exekall/src/exekall/_utils.py
+++ b/tools/exekall/src/exekall/_utils.py
@@ -558,7 +558,7 @@ def infer_mod_name(python_src, package_roots=None, excep_handler=None):
             else:
                 break
         else:
-            raise ValueError('Could not find {python_src} in any of the package roots: {package_roots}')
+            raise ValueError(f'Could not find {python_src} in any of the package roots: {package_roots}')
 
     # Pick the longest path of both
     paths = [module_path1, module_path2]