diff --git a/src/wireviz/Harness.py b/src/wireviz/Harness.py
index c4af2364f..9933f9e9a 100644
--- a/src/wireviz/Harness.py
+++ b/src/wireviz/Harness.py
@@ -298,7 +298,7 @@ def create_graph(self) -> Graph:
             html = []
 
             awg_fmt = ""
-            if cable.show_equiv:
+            if cable.show_equiv and isinstance(cable.gauge_unit, str):
                 # Only convert units we actually know about, i.e. currently
                 # mm2 and awg --- other units _are_ technically allowed,
                 # and passed through as-is.