diff --git a/src/ast.hpp b/src/ast.hpp
index be9dc0e34..5dd26f79c 100644
--- a/src/ast.hpp
+++ b/src/ast.hpp
@@ -27,6 +27,7 @@
 #include "ast_def_macros.hpp"
 #include "ast_fwd_decl.hpp"
 #include "source_map.hpp"
+#include "fn_utils.hpp"
 
 #include "sass.h"
 
@@ -913,8 +914,6 @@ namespace Sass {
   // Definitions for both mixins and functions. The two cases are distinguished
   // by a type tag.
   /////////////////////////////////////////////////////////////////////////////
-  struct Backtrace;
-  typedef Expression_Ptr (*Native_Function)(Env&, Env&, Context&, Signature, ParserState, Backtraces, SelectorStack);
   class Definition : public Has_Block {
   public:
     enum Type { MIXIN, FUNCTION };
diff --git a/src/ast_fwd_decl.hpp b/src/ast_fwd_decl.hpp
index 611995df9..26b330e09 100644
--- a/src/ast_fwd_decl.hpp
+++ b/src/ast_fwd_decl.hpp
@@ -426,7 +426,6 @@ namespace Sass {
   typedef std::vector<Selector_List_Obj> SelectorStack;
   typedef std::vector<Sass_Import_Entry> ImporterStack;
 
-  typedef const char* Signature;
   // only to switch implementations for testing
   #define environment_map std::map
 
diff --git a/src/fn_miscs.cpp b/src/fn_miscs.cpp
index 70bbd403d..938e3f153 100644
--- a/src/fn_miscs.cpp
+++ b/src/fn_miscs.cpp
@@ -146,7 +146,7 @@ namespace Sass {
       Expand expand(ctx, &d_env, &selector_stack);
       func->via_call(true); // calc invoke is allowed
       if (ff) func->func(ff);
-      return func->perform(&expand.eval);
+      return Cast<PreValue>(func->perform(&expand.eval));
     }
 
     ////////////////////
diff --git a/src/fn_utils.hpp b/src/fn_utils.hpp
index 4d00d904c..0ff39d8cf 100644
--- a/src/fn_utils.hpp
+++ b/src/fn_utils.hpp
@@ -8,16 +8,23 @@
 
 namespace Sass {
 
-  #define BUILT_IN(name) Expression_Ptr \
-    name(Env& env, Env& d_env, Context& ctx, Signature sig, ParserState pstate, Backtraces traces, SelectorStack selector_stack)
+  #define FN_PROTOTYPE \
+    Env& env, \
+    Env& d_env, \
+    Context& ctx, \
+    Signature sig, \
+    ParserState pstate, \
+    Backtraces& traces, \
+    SelectorStack& selector_stack
+
+  typedef const char* Signature;
+  typedef PreValue_Ptr (*Native_Function)(FN_PROTOTYPE);
+  #define BUILT_IN(name) PreValue_Ptr name(FN_PROTOTYPE)
 
   #define ARG(argname, argtype) get_arg<argtype>(argname, env, sig, pstate, traces)
-
   // special function for weird hsla percent (10px == 10% == 10 != 0.1)
   #define ARGVAL(argname) get_arg_val(argname, env, sig, pstate, traces) // double
 
-  typedef Expression_Ptr (*Native_Function)(Env&, Env&, Context&, Signature, ParserState, Backtraces, SelectorStack);
-
   Definition_Ptr make_native_function(Signature, Native_Function, Context& ctx);
   Definition_Ptr make_c_function(Sass_Function_Entry c_func, Context& ctx);