diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 4996a71c..3236bfdb 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -2540,15 +2540,18 @@ void DerivationGoal::runChild()
              */
             sandboxProfile += "(allow file-read* file-write* process-exec\n";
             for (auto & i : dirsInChroot) {
-                if (i.first != i.second)
+                if (i.first != i.second.source)
                     throw Error(format(
                         "can't map '%1%' to '%2%': mismatched impure paths not supported on Darwin")
-                        % i.first % i.second);
+                        % i.first % i.second.source);
 
                 string path = i.first;
                 struct stat st;
-                if (lstat(path.c_str(), &st))
+                if (lstat(path.c_str(), &st)) {
+                    if (i.second.optional && errno == ENOENT)
+                        continue;
                     throw SysError(format("getting attributes of path ‘%1%’") % path);
+                }
                 if (S_ISDIR(st.st_mode))
                     sandboxProfile += (format("\t(subpath \"%1%\")\n") % path).str();
                 else