diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 341dedfdf..2391e14a9 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -73,6 +73,13 @@ std::map<std::string, std::string> getEnv()
 }
 
 
+void clearEnv()
+{
+    for (auto & name : getEnv())
+        unsetenv(name.first.c_str());
+}
+
+
 Path absPath(Path path, Path dir)
 {
     if (path[0] != '/') {
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index 47e02bc89..c5c537ee6 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -32,6 +32,9 @@ string getEnv(const string & key, const string & def = "");
 /* Get the entire environment. */
 std::map<std::string, std::string> getEnv();
 
+/* Clear the environment. */
+void clearEnv();
+
 /* Return an absolutized path, resolving paths relative to the
    specified directory, or the current directory otherwise.  The path
    is also canonicalised. */
diff --git a/src/nix/run.cc b/src/nix/run.cc
index 822654daf..d04e106e0 100644
--- a/src/nix/run.cc
+++ b/src/nix/run.cc
@@ -16,8 +16,6 @@ using namespace nix;
 
 std::string chrootHelperName = "__run_in_chroot";
 
-extern char * * environ;
-
 struct CmdRun : InstallablesCommand
 {
     std::vector<std::string> command = { "bash" };
@@ -109,7 +107,7 @@ struct CmdRun : InstallablesCommand
                 if (s) kept[var] = s;
             }
 
-            environ = nullptr;
+            clearEnv();
 
             for (auto & var : kept)
                 setenv(var.first.c_str(), var.second.c_str(), 1);