diff --git a/units.py b/units.py
index f811fb9..8ab046a 100644
--- a/units.py
+++ b/units.py
@@ -43,7 +43,7 @@ def unit_str(unit, base=None, prefix="", format=" [{unit}]"):
         return ""
     elif base is not None:
         coeff = unit.express(base)
-        u_str = unit_str(base, prefix=convert_exp(coeff) + " ")
+        return unit_str(base, prefix=convert_exp(coeff) + " ")
     elif len(unit.latex) > 0:
         if "." in unit.latex or "^" in unit.latex:
             base_str = ".".join(map(parse_exp_unit, unit.name.split(".")))