diff --git a/spec/ruby/optional/capi/ext/rubyspec.h b/spec/ruby/optional/capi/ext/rubyspec.h
index e5a90bfc88..63691d62fd 100644
--- a/spec/ruby/optional/capi/ext/rubyspec.h
+++ b/spec/ruby/optional/capi/ext/rubyspec.h
@@ -572,6 +572,7 @@
 #define HAVE_RB_STRING                     1
 #define HAVE_SAFE_STRING_VALUE             1
 #define HAVE_RB_STRING_VALUE_CSTR          1
+#define HAVE_RB_STR_MODIFY                 1
 
 /* Struct */
 #define HAVE_RB_STRUCT_AREF                1
diff --git a/spec/ruby/optional/capi/ext/string_spec.c b/spec/ruby/optional/capi/ext/string_spec.c
index a6ce02b25c..6650034ef1 100644
--- a/spec/ruby/optional/capi/ext/string_spec.c
+++ b/spec/ruby/optional/capi/ext/string_spec.c
@@ -504,6 +504,13 @@ static VALUE string_spec_rb_string_value_cstr(VALUE self, VALUE str) {
 }
 #endif
 
+#ifdef HAVE_RB_STR_MODIFY
+static VALUE string_spec_rb_str_modify(VALUE self, VALUE str) {
+  rb_str_modify(str);
+  return str;
+}
+#endif
+
 void Init_string_spec(void) {
   VALUE cls;
   cls = rb_define_class("CApiStringSpecs", rb_cObject);
@@ -744,6 +751,10 @@ void Init_string_spec(void) {
 #ifdef HAVE_RB_STRING_VALUE_CSTR
   rb_define_method(cls, "rb_string_value_cstr", string_spec_rb_string_value_cstr, 1);
 #endif
+
+#ifdef HAVE_RB_STR_MODIFY
+  rb_define_method(cls, "rb_str_modify", string_spec_rb_str_modify, 1);
+#endif
 }
 #ifdef __cplusplus
 }
diff --git a/spec/ruby/optional/capi/string_spec.rb b/spec/ruby/optional/capi/string_spec.rb
index 2344dd37d7..cfd60c2593 100644
--- a/spec/ruby/optional/capi/string_spec.rb
+++ b/spec/ruby/optional/capi/string_spec.rb
@@ -36,7 +36,8 @@ describe "C-API String function" do
     before :each do
       # Make a completely new copy of the string
       # for every example (#dup doesn't cut it).
-      @str = "abcdefghij"[0..-1]
+      @str = "abcdefghij"
+      @s.rb_str_modify(@str)
     end
 
     it "reduces the size of the string" do