diff --git a/units.py b/units.py
index 8a2fed3..f811fb9 100644
--- a/units.py
+++ b/units.py
@@ -41,7 +41,7 @@ def unit_str(unit, base=None, prefix="", format=" [{unit}]"):
     """
     if unit == U.none:
         return ""
-    elif not base is None:
+    elif base is not None:
         coeff = unit.express(base)
         u_str = unit_str(base, prefix=convert_exp(coeff) + " ")
     elif len(unit.latex) > 0:
@@ -51,11 +51,8 @@ def unit_str(unit, base=None, prefix="", format=" [{unit}]"):
         else:
             u_str = r"${}{}$".format(prefix, unit.latex.replace("text", "math"))
     elif len(unit.name) > 0:
-        try:
-            base_str = ".".join(map(parse_exp_unit, unit.name.split(".")))
-            u_str = r"${}{}$".format(prefix, base_str)
-        except:
-            u_str = r"${}{}$".format(prefix, unit.name)
+        base_str = ".".join(map(parse_exp_unit, unit.name.split(".")))
+        u_str = r"${}{}$".format(prefix, base_str)
     else:
         base_str = ".".join(
             map(parse_exp_unit, unit._decompose_base_units().split("."))
@@ -92,3 +89,10 @@ U.ssfrG = create_unit(
 U.uG = create_unit(
     "μG", base_unit=1e-10 * U.T, descr="Micro Gauss", latex="\\mu\\mathrm{G}"
 )
+
+U.ssfrK = create_unit(
+    "Msun.year^-1.kpc^-2",
+    base_unit=U.Msun / U.year / U.kpc ** 2,
+    descr="Surfacic SFR",
+    latex="\mathrm{M}_{\odot}.\mathrm{yr}^{-1}.\mathrm{kpc}^{-2}",
+)