diff --git a/ext/ffi_c/Call.c b/ext/ffi_c/Call.c
index e70e5f8a4..ce0539dec 100644
--- a/ext/ffi_c/Call.c
+++ b/ext/ffi_c/Call.c
@@ -43,6 +43,9 @@
 #endif
 #include <errno.h>
 #include <ruby.h>
+#if defined(HAVE_RUBY_THREAD_H)
+#include <ruby/thread.h>
+#endif
 #if defined(HAVE_NATIVETHREAD) && (defined(HAVE_RB_THREAD_BLOCKING_REGION) || defined(HAVE_RB_THREAD_CALL_WITHOUT_GVL)) && !defined(_WIN32)
 #  include <signal.h>
 #  include <pthread.h>
@@ -317,7 +320,7 @@ rbffi_SetupCallParams(int argc, VALUE* argv, int paramCount, Type** paramTypes,
     }
 }
 
-static VALUE
+static void *
 call_blocking_function(void* data)
 {
     rbffi_blocking_call_t* b = (rbffi_blocking_call_t *) data;
@@ -325,7 +328,7 @@ call_blocking_function(void* data)
     ffi_call(&b->cif, FFI_FN(b->function), b->retval, b->ffiValues);
     b->frame->has_gvl = true;
 
-    return Qnil;
+    return NULL;
 }
 
 VALUE
diff --git a/ext/ffi_c/Function.c b/ext/ffi_c/Function.c
index d9ba5c165..7047116bb 100644
--- a/ext/ffi_c/Function.c
+++ b/ext/ffi_c/Function.c
@@ -47,6 +47,9 @@
 # endif
 #endif
 #include <ruby.h>
+#if defined(HAVE_RUBY_THREAD_H)
+#include <ruby/thread.h>
+#endif
 
 #include <ffi.h>
 #if defined(HAVE_NATIVETHREAD) && !defined(_WIN32)
@@ -554,7 +557,7 @@ struct async_wait {
     bool stop;
 };
 
-static VALUE async_cb_wait(void *);
+static void * async_cb_wait(void *);
 static void async_cb_stop(void *);
 
 #if defined(HAVE_RB_THREAD_BLOCKING_REGION) || defined(HAVE_RB_THREAD_CALL_WITHOUT_GVL)
@@ -643,7 +646,7 @@ async_cb_event(void* unused)
 #endif
 
 #ifdef _WIN32
-static VALUE
+static void *
 async_cb_wait(void *data)
 {
     struct async_wait* w = (struct async_wait *) data;
@@ -665,7 +668,7 @@ async_cb_wait(void *data)
 
     LeaveCriticalSection(&async_cb_lock);
 
-    return Qnil;
+    return NULL;
 }
 
 static void
@@ -680,7 +683,7 @@ async_cb_stop(void *data)
 }
 
 #else
-static VALUE
+static void *
 async_cb_wait(void *data)
 {
     struct async_wait* w = (struct async_wait *) data;
@@ -700,7 +703,7 @@ async_cb_wait(void *data)
 
     pthread_mutex_unlock(&async_cb_mutex);
 
-    return Qnil;
+    return NULL;
 }
 
 static void
diff --git a/ext/ffi_c/extconf.rb b/ext/ffi_c/extconf.rb
index 23f466939..45ab9770f 100644
--- a/ext/ffi_c/extconf.rb
+++ b/ext/ffi_c/extconf.rb
@@ -26,6 +26,7 @@
   end
 
   have_header('shlwapi.h')
+  have_header('ruby/thread.h') # for compat with ruby < 2.0
   have_func('rb_thread_blocking_region')
   have_func('rb_thread_call_with_gvl')
   have_func('rb_thread_call_without_gvl')