diff --git a/pcbnew/python/swig/units.i b/pcbnew/python/swig/units.i
index 441eacac81..fd560d50a2 100644
--- a/pcbnew/python/swig/units.i
+++ b/pcbnew/python/swig/units.i
@@ -36,24 +36,32 @@
             return float(iu) / float(IU_PER_MM)
         elif type(iu) in [wxPoint,wxSize]:
             return tuple(map(ToMM,iu))
+        else:
+            raise TypeError("ToMM() excpects int, float, wxPoint or wxSize, instead got type " + str(type(iu)))
 
     def FromMM(mm):
         if type(mm) in [int,float]:
             return int(float(mm) * float(IU_PER_MM))
         elif type(mm) in [wxPoint,wxSize]:
             return tuple(map(FromMM,mm))
+        else:
+            raise TypeError("FromMM() expects int, float, wxPoint or wxSize, instead got type " + str(type(mm)))
 
     def ToMils(iu):
         if type(iu) in [int,float]:
             return float(iu) / float(IU_PER_MILS)
         elif type(iu) in [wxPoint,wxSize]:
             return tuple(map(ToMils,iu))
+        else:
+            raise TypeError("ToMils() excpects int, float, wxPoint or wxSize, instead got type " + str(type(iu)))
 
     def FromMils(mils):
         if type(mils) in [int,float]:
             return int(float(mils)*float(IU_PER_MILS))
         elif type(mils) in [wxPoint,wxSize]:
             return tuple(map(FromMils,mils))
+        else:
+            raise TypeError("FromMils() excpects int, float, wxPoint or wxSize, instead got type " + str(type(mils)))
 
     def PutOnGridMM(value, gridSizeMM):
         thresh = FromMM(gridSizeMM)