diff --git a/src/libutil/json.cc b/src/libutil/json.cc
index 619b1d631..ecc3fdfe5 100644
--- a/src/libutil/json.cc
+++ b/src/libutil/json.cc
@@ -29,6 +29,11 @@ void toJSON(std::ostream & str, const char * s)
     if (!s) str << "null"; else toJSON(str, s, s + strlen(s));
 }
 
+void toJSON(std::ostream & str, unsigned long long n)
+{
+    str << n;
+}
+
 void toJSON(std::ostream & str, unsigned long n)
 {
     str << n;
diff --git a/src/libutil/json.hh b/src/libutil/json.hh
index 852000a51..aec456845 100644
--- a/src/libutil/json.hh
+++ b/src/libutil/json.hh
@@ -9,6 +9,7 @@ namespace nix {
 void toJSON(std::ostream & str, const char * start, const char * end);
 void toJSON(std::ostream & str, const std::string & s);
 void toJSON(std::ostream & str, const char * s);
+void toJSON(std::ostream & str, unsigned long long n);
 void toJSON(std::ostream & str, unsigned long n);
 void toJSON(std::ostream & str, long n);
 void toJSON(std::ostream & str, double f);
diff --git a/src/nix/path-info.cc b/src/nix/path-info.cc
index b25e60db6..d8b6232d4 100644
--- a/src/nix/path-info.cc
+++ b/src/nix/path-info.cc
@@ -1,11 +1,8 @@
 #include "command.hh"
 #include "shared.hh"
 #include "store-api.hh"
-
-
 #include "json.hh"
 
-
 #include <iomanip>
 #include <algorithm>
 
@@ -60,8 +57,8 @@ struct CmdPathInfo : StorePathsCommand
         for (auto & storePath : storePaths)
             pathLen = std::max(pathLen, storePath.size());
 
-        auto getClosureSize = [&](const Path & storePath) {
-            size_t totalSize = 0;
+        auto getClosureSize = [&](const Path & storePath) -> unsigned long long {
+            unsigned long long totalSize = 0;
             PathSet closure;
             store->computeFSClosure(storePath, closure, false, false);
             for (auto & p : closure)