diff --git a/ext/jsonnet/callbacks.c b/ext/jsonnet/callbacks.c
index 733d4cf..800e503 100644
--- a/ext/jsonnet/callbacks.c
+++ b/ext/jsonnet/callbacks.c
@@ -130,8 +130,7 @@ import_callback_entrypoint(void *ctx, const char *base, const char *rel, char **
     if (state) {
 	VALUE msg = rescue_callback(state, "cannot import %s from %s", rel, base);
 #ifdef HAVE_JSONNET_IMPORT_CALLBACK_0_19
-	*buf = rubyjsonnet_str_to_cstr(vm->vm, msg);
-	*buflen = RSTRING_LEN(msg);
+	*buf = rubyjsonnet_str_to_ptr(vm->vm, msg, buflen);
 	return 1;
 #else
 	*success = 0;
@@ -142,8 +141,7 @@ import_callback_entrypoint(void *ctx, const char *base, const char *rel, char **
     result = rb_Array(result);
     *found_here = rubyjsonnet_str_to_cstr(vm->vm, rb_ary_entry(result, 1));
 #ifdef HAVE_JSONNET_IMPORT_CALLBACK_0_19
-    *buf = rubyjsonnet_str_to_cstr(vm->vm, rb_ary_entry(result, 0));
-    *buflen = RSTRING_LEN(rb_ary_entry(result, 0));
+    *buf = rubyjsonnet_str_to_ptr(vm->vm, rb_ary_entry(result, 0), buflen);
     return 0;
 #else
     *success = 1;
diff --git a/ext/jsonnet/helpers.c b/ext/jsonnet/helpers.c
index 5958d60..966770b 100644
--- a/ext/jsonnet/helpers.c
+++ b/ext/jsonnet/helpers.c
@@ -34,6 +34,13 @@ rubyjsonnet_assert_asciicompat(VALUE str)
 
 /**
  * Allocates a C string whose content is equal to \c str with jsonnet_realloc.
+ *
+ * Note that this function does not allow NUL characters in the string.
+ * You should use rubyjsonnet_str_to_ptr() if you want to handle NUL characters.
+ *
+ * @param[in] vm a Jsonnet VM
+ * @param[in] str a String-like object
+ * @return the allocated C string
  */
 char *
 rubyjsonnet_str_to_cstr(struct JsonnetVm *vm, VALUE str)
@@ -44,6 +51,25 @@ rubyjsonnet_str_to_cstr(struct JsonnetVm *vm, VALUE str)
     return buf;
 }
 
+/**
+ * Allocates a byte sequence whose content is equal to \c str with jsonnet_realloc.
+ *
+ * @param[in] vm a Jsonnet VM
+ * @param[in] str a String-like object
+ * @param[out] buflen the length of the allocated buffer
+ * @return the allocated buffer
+ */
+char *
+rubyjsonnet_str_to_ptr(struct JsonnetVm *vm, VALUE str, size_t *buflen)
+{
+    StringValue(str);
+    size_t len = RSTRING_LEN(str);
+    char *buf = jsonnet_realloc(vm, NULL, len);
+    memcpy(buf, RSTRING_PTR(str), len);
+    *buflen = len;
+    return buf;
+}
+
 /**
  * @return a human readable string which contains the class name of the
  *   exception and its message. It might be nil on failure
diff --git a/ext/jsonnet/ruby_jsonnet.h b/ext/jsonnet/ruby_jsonnet.h
index aa91c72..b33b57e 100644
--- a/ext/jsonnet/ruby_jsonnet.h
+++ b/ext/jsonnet/ruby_jsonnet.h
@@ -33,6 +33,7 @@ struct JsonnetJsonValue *rubyjsonnet_obj_to_json(struct JsonnetVm *vm, VALUE obj
 
 rb_encoding *rubyjsonnet_assert_asciicompat(VALUE str);
 char *rubyjsonnet_str_to_cstr(struct JsonnetVm *vm, VALUE str);
+char *rubyjsonnet_str_to_ptr(struct JsonnetVm *vm, VALUE str, size_t *buflen);
 VALUE rubyjsonnet_format_exception(VALUE exc);
 int rubyjsonnet_jump_tag(const char *exc_mesg);
 
diff --git a/test/test_vm.rb b/test/test_vm.rb
index 27ca116..45ff770 100644
--- a/test/test_vm.rb
+++ b/test/test_vm.rb
@@ -287,6 +287,26 @@ class TestVM < Test::Unit::TestCase
     assert_equal expected, JSON.parse(result)
   end
 
+  test "Jsonnet::VM#import_callback allows NUL char in Jsonnet v0.19 or later" do
+    return unless Jsonnet.libversion >= "v0.19"
+
+    example_str = "\x0\x1".force_encoding(Encoding::BINARY)
+
+    vm = Jsonnet::VM.new
+    vm.import_callback = ->(base, rel) {
+      case [base, rel]
+      when ['/path/to/base/', 'foo.bin']
+        return "\x0\x1".force_encoding(Encoding::BINARY), '/path/to/base/foo.bin'
+      else
+        raise Errno::ENOENT, "#{rel} at #{base}"
+      end
+    }
+    result = vm.evaluate(<<-EOS, filename: "/path/to/base/example.jsonnet")
+      importbin "foo.bin"
+    EOS
+    assert_equal [0, 1], JSON.parse(result)
+  end
+
   test "Jsonnet::VM#evaluate returns an error if customized import callback raises an exception" do
     vm = Jsonnet::VM.new
     called = false