Error installing frank-cucumber. Problem with JSON 1.8.1?

[reach2jeyan]

User-MacBook-Pro:~ user$ sudo gem install frank-cucumber
Building native extensions. This could take a while...
ERROR:  Error installing frank-cucumber:
	ERROR: Failed to build gem native extension.

    current directory: /Users/user.rvm/rubies/ruby-2.6.3/lib/ruby/gems/2.6.0/gems/json-1.8.1/ext/json/ext/generator
/Users/user/.rvm/rubies/ruby-2.6.3/bin/ruby -I /Users/user/.rvm/rubies/ruby-2.6.3/lib/ruby/2.6.0 -r ./siteconf20190425-19021-s115r2.rb extconf.rb
creating Makefile

current directory: /Users/user/.rvm/rubies/ruby-2.6.3/lib/ruby/gems/2.6.0/gems/json-1.8.1/ext/json/ext/generator
make "DESTDIR=" clean

current directory: /Users/user.rvm/rubies/ruby-2.6.3/lib/ruby/gems/2.6.0/gems/json-1.8.1/ext/json/ext/generator
make "DESTDIR="
compiling generator.c
In file included from generator.c:1:
./../fbuffer/fbuffer.h:175:47: error: too few arguments provided to function-like macro invocation
    VALUE result = rb_str_new(FBUFFER_PAIR(fb));
                                              ^
/Users/user.rvm/rubies/ruby-2.6.3/include/ruby-2.6.0/ruby/intern.h:814:9: note: macro 'rb_str_new' defined here
#define rb_str_new(str, len) RB_GNUC_EXTENSION_BLOCK(   \
        ^
In file included from generator.c:1:
./../fbuffer/fbuffer.h:175:11: warning: incompatible pointer to integer conversion initializing 'VALUE' (aka 'unsigned long') with an expression of type 'VALUE (const char *, long)' (aka 'unsigned long (const char *, long)') [-Wint-conversion]
    VALUE result = rb_str_new(FBUFFER_PAIR(fb));
          ^        ~~~~~~~~~~
generator.c:840:25: error: use of undeclared identifier 'rb_cFixnum'
    } else if (klass == rb_cFixnum) {
                        ^
generator.c:842:25: error: use of undeclared identifier 'rb_cBignum'
    } else if (klass == rb_cBignum) {
                        ^
1 warning and 3 errors generated.
make: *** [generator.o] Error 1

make failed, exit code 2

Gem files will remain installed in /Users/user/.rvm/rubies/ruby-2.6.3/lib/ruby/gems/2.6.0/gems/json-1.8.1 for inspection.
Results logged to /Users/user/.rvm/rubies/ruby-2.6.3/lib/ruby/gems/2.6.0/extensions/x86_64-darwin-18/2.6.0/json-1.8.1/gem_make.out