diff --git a/CHERICC_Fat.bsv b/CHERICC_Fat.bsv
index 423df64..4175d96 100644
--- a/CHERICC_Fat.bsv
+++ b/CHERICC_Fat.bsv
@@ -904,7 +904,7 @@ instance CHERICap #(CapMem, OTypeW, FlagsW, CapAddressW, CapW, TSub#(MW, 3));
   function setBoundsCombined = error("feature not implemented for this cap type");
   function nullWithAddr = error("feature not implemented for this cap type");
   function almightyCap = error("feature not implemented for this cap type");
-  function nullCap = error("feature not implemented for this cap type");
+  function nullCapFromDummy = error("feature not implemented for this cap type");
   function validAsType = error("feature not implemented for this cap type");
   function fromMem = error("feature not implemented for this cap type");
   function toMem = error("feature not implemented for this cap type");
@@ -1030,7 +1030,7 @@ instance CHERICap #(CapReg, OTypeW, FlagsW, CapAddressW, CapW, TSub#(MW, 3));
 
   function almightyCap = defaultCapFat;
 
-  function nullCap = null_cap;
+  function nullCapFromDummy (x) = null_cap;
 
   function Bool validAsType (CapReg dummy, Bit#(CapAddressW) checkType);
     UInt#(CapAddressW) checkTypeUnsigned = unpack(checkType);
@@ -1120,7 +1120,7 @@ instance CHERICap #(CapPipe, OTypeW, FlagsW, CapAddressW, CapW, TSub#(MW, 3));
     return CapPipe {capFat: res, tempFields: getTempFields(res)};
   endfunction
 
-  function nullCap;
+  function nullCapFromDummy (x);
     CapReg res = nullCap;
     return CapPipe {capFat: res, tempFields: getTempFields(res)};
   endfunction
diff --git a/CHERICap.bsv b/CHERICap.bsv
index e5857f8..9fc9981 100644
--- a/CHERICap.bsv
+++ b/CHERICap.bsv
@@ -170,10 +170,13 @@ typeclass CHERICap#(type t, numeric type ot, numeric type flg, numeric type n, n
   // Returns a null cap with an address set to the argument
   function t nullWithAddr (Bit#(n) addr);
 
+  // Workaround to allow null cap to be derived in default implementations
+  function t nullCapFromDummy(t dummy);
+
   // Return the maximally permissive capability (initial register state)
   function t almightyCap;
   // Return the null capability
-  function t nullCap;
+  function t nullCap = nullCapFromDummy(?);
 
   // Check if a type is valid
   function Bool validAsType (t dummy, Bit#(n) checkType);
@@ -196,12 +199,12 @@ typeclass CHERICap#(type t, numeric type ot, numeric type flg, numeric type n, n
 
   // TODO the following do not compile due to "not enough explicit type information"
   // Get representable alignment mask
-  // function Bit#(n) getRepresentableAlignmentMask (t dummy, Bit#(n) length_request) =
-  //   setBoundsCombined(nullCap, length_request).mask;
+  function Bit#(n) getRepresentableAlignmentMask (t dummy, Bit#(n) length_request) =
+    setBoundsCombined(nullCapFromDummy(dummy), length_request).mask;
 
   // Get representable length
-  // function Bit#(n) getRepresentableLength (t dummy, Bit#(n) length_request) =
-  //   setBoundsCombined(nullCap, length_request).length;
+  function Bit#(n) getRepresentableLength (t dummy, Bit#(n) length_request) =
+    setBoundsCombined(nullCapFromDummy(dummy), length_request).length;
 
   // Assert that the encoding is valid
   function Bool isDerivable (t cap);