diff --git a/features/steps/thens/attributes.py b/features/steps/thens/attributes.py
index f113d583..3da93fc2 100644
--- a/features/steps/thens/attributes.py
+++ b/features/steps/thens/attributes.py
@@ -140,6 +140,8 @@ def step_impl(context, inst, constraint, num):
 @gherkin_ifc.step('The characters must be within the official encoding character set')
 def step_impl(context, inst):
     valid_chars = set("0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz_$")
+    if not isinstance(inst, str):
+        return
     invalid_guid_chars = [char for char in inst if char not in valid_chars]
     if invalid_guid_chars:
         yield ValidationOutcome(inst=inst, expected="^[0-9A-Za-z_$]+$", observed=invalid_guid_chars, severity=OutcomeSeverity.ERROR)