diff --git a/src/wallet/asyncrpcoperation_mergetoaddress.cpp b/src/wallet/asyncrpcoperation_mergetoaddress.cpp
index fa823f50a..a85f3315c 100644
--- a/src/wallet/asyncrpcoperation_mergetoaddress.cpp
+++ b/src/wallet/asyncrpcoperation_mergetoaddress.cpp
@@ -753,8 +753,14 @@ UniValue AsyncRPCOperation_mergetoaddress::perform_joinsplit(
     // Generate the proof, this can take over a minute.
     boost::array<libzcash::JSInput, ZC_NUM_JS_INPUTS> inputs{info.vjsin[0], info.vjsin[1]};
     boost::array<libzcash::JSOutput, ZC_NUM_JS_OUTPUTS> outputs{info.vjsout[0], info.vjsout[1]};
+    #ifdef __LP64__
+    boost::array<uint64_t, ZC_NUM_JS_INPUTS> inputMap;
+    boost::array<uint64_t, ZC_NUM_JS_OUTPUTS> outputMap;
+    #else
     boost::array<size_t, ZC_NUM_JS_INPUTS> inputMap;
     boost::array<size_t, ZC_NUM_JS_OUTPUTS> outputMap;
+    #endif
+
 
     uint256 esk; // payment disclosure - secret