diff --git a/src/libcmd/command.cc b/src/libcmd/command.cc
index 56288665a..18aa82577 100644
--- a/src/libcmd/command.cc
+++ b/src/libcmd/command.cc
@@ -93,10 +93,6 @@ EvalCommand::EvalCommand()
     });
 }
 
-extern std::function<void(const Error * error, const Env & env, const Expr & expr)> debuggerHook;
-
-
-
 EvalCommand::~EvalCommand()
 {
     if (evalState)
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index f21919598..6758677ca 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -37,8 +37,6 @@
 
 namespace nix {
 
-std::function<void(const Error * error, const Env & env, const Expr & expr)> debuggerHook;
-
 static char * allocString(size_t size)
 {
     char * t;
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index e1d117c36..5f4556053 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -25,7 +25,6 @@ enum RepairFlag : bool;
 
 typedef void (* PrimOpFun) (EvalState & state, const Pos & pos, Value * * args, Value & v);
 
-extern std::function<void(const Error * error, const Env & env, const Expr & expr)> debuggerHook;
 void printStaticEnvBindings(const Expr &expr);
 void printStaticEnvBindings(const StaticEnv &se, int lvl = 0);
 
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index e09bd9484..add65c1a2 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -6,6 +6,9 @@
 
 namespace nix {
 
+/* Launch the nix debugger */
+
+std::function<void(const Error * error, const Env & env, const Expr & expr)> debuggerHook;
 
 /* Displaying abstract syntax trees. */