diff --git a/SimPEG/EM/FDEM/FDEM.py b/SimPEG/EM/FDEM/FDEM.py
index 5c2683b2..3c2183b1 100644
--- a/SimPEG/EM/FDEM/FDEM.py
+++ b/SimPEG/EM/FDEM/FDEM.py
@@ -36,7 +36,7 @@ class BaseFDEMProblem(BaseEMProblem):
     surveyPair = SurveyFDEM
     fieldsPair = Fields
 
-    def fields(self, m=None):
+    def fields(self, m):
         """
         Solve the forward problem for the fields.