diff --git a/ext/debug/debug.c b/ext/debug/debug.c
index 9eaac2bbd..241f2c0f0 100644
--- a/ext/debug/debug.c
+++ b/ext/debug/debug.c
@@ -180,13 +180,17 @@ iseq_last_line(VALUE iseqw)
 }
 #endif
 
+#ifdef CRUBY
 void Init_iseq_collector(void);
+#endif
 
 void
 Init_debug(void)
 {
+#ifdef CRUBY
     VALUE rb_mRubyVM = rb_const_get(rb_cObject, rb_intern("RubyVM"));
     VALUE rb_cISeq = rb_const_get(rb_mRubyVM, rb_intern("InstructionSequence"));
+#endif
     rb_mDebugger = rb_const_get(rb_cObject, rb_intern("DEBUGGER__"));
     rb_cFrameInfo = rb_const_get(rb_mDebugger, rb_intern("FrameInfo"));
 
@@ -210,5 +214,7 @@ Init_debug(void)
     rb_define_method(rb_cISeq, "last_line", iseq_last_line, 0);
 #endif
 
+#ifdef CRUBY
     Init_iseq_collector();
+#endif
 }
diff --git a/ext/debug/extconf.rb b/ext/debug/extconf.rb
index b4e78fbe3..dc2964d2c 100644
--- a/ext/debug/extconf.rb
+++ b/ext/debug/extconf.rb
@@ -4,6 +4,7 @@
 $distcleanfiles << "debug_version.h"
 
 if defined? RubyVM
+  $defs << '-DCRUBY'
   $defs << '-DHAVE_RB_ISEQ_PARAMETERS'
   $defs << '-DHAVE_RB_ISEQ_CODE_LOCATION'
 
diff --git a/ext/debug/iseq_collector.c b/ext/debug/iseq_collector.c
index f87c4f978..3504dee6d 100644
--- a/ext/debug/iseq_collector.c
+++ b/ext/debug/iseq_collector.c
@@ -1,3 +1,4 @@
+#ifdef CRUBY
 #include <ruby/ruby.h>
 
 VALUE rb_iseqw_new(VALUE v);
@@ -89,3 +90,4 @@ Init_iseq_collector(void)
     rb_define_singleton_method(rb_mObjSpace, "each_iseq", each_iseq, 0);
     rb_define_singleton_method(rb_mObjSpace, "count_iseq", count_iseq, 0);
 }
+#endif